I refactored and structuralized RSpec test cases of AnnotateModels for readability and scalability because it was too complex to read.

cf. #718 
In this PR, I refactored test cases of some methods in `AnnotateModels`.
I will refactor test cases of other methods in another PR.

I refactored two methods in AnnotateModels::Helpers for readability.

I refactored `AnnotateRoutes.routes_exists?` and methods using this.
The points are as follows.

*   Removing `puts` in `AnnotateRoutes.routes_exists?`
*   Using `File.exist?` instead of `File.exists?` because `File.exists?` is deprecated
*   Renaming `AnnotateRoutes.routes_exists?` to `AnnotateRoutes.routes_file_exists?` in order to make the name of method more explanatory

* Refactor AnnotateRoutes.rewrite_contents and .remove_annotations

* Refactor AnnotateRoutes.remove_annotations and .strip_on_removal

* Remove Annotate.#loaded_tasks= and .loaded_tasks

* Remove unnecessary comment for Rubocop

* Make Annotate.load_requires private

* Refactor Annotate.load_tasks

I refactored `AnnotateRoutes.header` for performance, and add `AnnotateRoutes.comment` to make code DRY.

As the change log shows, I froze the constant `HEADER_ROW` in accordance with the normal Ruby style regulation.

I added `AnnotateModels::MAGIC_COMMENT_MATCHER` and `AnnotateRoutes::MAGIC_COMMENT_MATCHER` instad of `AnnotateModels.magic_comment_matcher` and `AnnotateRoutes.magic_comment_matcher`.

I think that they should be constants because the result of Regexp is always fixed.

Before I commited, the method `AnnotateRoutes.where_header_found` returned an array of `real_content` and `header_found_at`.

I fixed the name of method and variable because they did not match.

I renamed the method `real_content_and_header_position` and unified related variable names to `header_position`.

The integration tests require RVM to run. Without them they can't run.

Added Dockerfile to make it easier to run the project in an isolated environment.

This change converts .all_options into a constant and moves it into Annotate::Constants. It also changes usages of .all_options.

Replace soft-deprecated constant `HashWithIndifferentAccess` to `ActiveSupport::HashWithIndifferentAccess` (#699)

Removing `TODO.md` as prior to renaming it hasn't been touched in 5 years. 

See https://github.com/ctran/annotate_models/commits/v3.0.3/TODO.rdoc

* Fix instances of `additional file patterns` in README

Other CLI flags are using dash, so using `--additional-file-patterns` notation to keep it similar to others.

* Tidy: Replace `*` with `*`

Doing some tidying now that we switched from RDoc to markdown.

Same problem returned by @janosrusiczki in #430

Spatial columns with activerecord-postgis-adapter (5.2.2) are not displayed nice

```
# name           :string(3)
# active         :boolean          default(FALSE), not null
# geom           :geometry({:srid= geometry, 4326
```

That's come from activerecord-postgis-adapter, with redefined columns as such :

``` 
#<ActiveRecord::ConnectionAdapters::PostGIS::SpatialColumn 
  @sql_type="geometry(Geometry,4326)",
  @geo_type="Geometry",
  @geometric_type=RGeo::Feature::Geometry,
  @srid=4326,
  @limit={:srid=>4326, :type=>"geometry"}
  [...]
>
```

This fix displays them like this :

```
# name           :string(3)
# active         :boolean          default(FALSE), not null
# geometry       :geometry         geometry, 4326
```

Another possibility would have been to display them as below, but it involves a lot of extra-spaces for other columns.

```
# name           :string(3)
# active         :boolean                   default(FALSE), not null
# geometry       :geometry(Geometry, 4326)
```

* Convert documentation files to Markdown
* Add notice about models not being annotated by default to changelog in 3.0

Looks like it's breaking in Github CI because Ruby 2.3.X is no longer supported by `actions/setup-ruby@v1`. Ruby 2.3.X has stopped being supported since March of 2019, so I've removed Ruby 2.3.X versions from both Github and Travis CI.

It's also breaking in Travis CI because it's timing out in an interactive prompt when trying to do `gem update --system`. This is due to changes to [Ruby Gems 3.1.1](https://github.com/rubygems/rubygems/issues/3036). This has been fixed.

Failing Github CI build: https://github.com/ctran/annotate_models/runs/335456181
Failing Travis build: https://api.travis-ci.org/v3/job/625759725/log.txt

ActiveAdmin defaults to false.  Add `active_admin` option so people using this gem can more easily enable annotating files in `app/admin`.

Rails 6 has dropped name from ActiveRecord::ConnectionAdapters::ForeignKeyDefinition
for Sqlite3 adapter:

https://github.com/rails/rails/blob/f2df77709f7e536aaf4d6f984ff21a49d44d34c1/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb\#L305-L311

This has caused issues with getting foreign key info and blowing up
when trying to annotate models with foreign keys. This commit
adds one additional check for name presence and uses column
attribute from options instead.

Closes #620.

Updates the readme to include a note about upgrading the gem to version 3 and that it can default to doing nothing.

Reverted tag to to use `v*` prefix instead of `release/v*`.

Bumps [loofah](https://github.com/flavorjones/loofah) from 2.3.0 to 2.3.1.
- [Release notes](https://github.com/flavorjones/loofah/releases)
- [Changelog](https://github.com/flavorjones/loofah/blob/master/CHANGELOG.md)
- [Commits](https://github.com/flavorjones/loofah/compare/v2.3.0...v2.3.1)
Signed-off-by: dependabot[bot] <support@github.com>

Setting the default to annotate models as it is the main feature of the gem

Based on the official docs:

https://help.github.com/en/articles/configuring-a-workflow#adding-a-workflow-status-badge-to-your-repository

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.6.6.2 to 1.10.4.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sparklemotion/nokogiri/compare/v1.6.6.2...v1.10.4)
Signed-off-by: dependabot[bot] <support@github.com>