Refactor test cases for entire rewrite in the future.

Adds integration tests to the project that's able to run in CI across different ruby versions.

* Adds a Rails 5.2.4.1 app
* Adds a Rails 6.0.2.1 app

Fixes #733 

Refactor of test cases for AnnotateRoutes.remove_annotations after #736.

Refactor test cases for AnnotateModels.get_schema_info with Globalize gem

* Fixed CHANGELOG.md to add link to each PR

* Fixed link to markdown format

The integration tests were removed since they were outdated and not working. These are dead files so let's remove them.

#725 #744

Helper methods were moved into Annotate::Helpers but instances in bin/annotate had not been updated. Because of this, bin/annotate would throw NoMethodError.

There's some left over files from #725, this change cleans up the remaining files in `spec/integration/`.

`spec/integration` contains a lot of files that were used for integration tests. They stopped were [disabled in 2014](https://github.com/ctran/annotate_models/commit/9540121243da58cad3ab3c0ba444f22445a995ed#diff-78ddf877ecc2a9344997ef077a77955a) and haven't been working since.

Removing because those tests don't run, don't work, and are outdated. The plan is to re-introduce integration tests sometime in the future.

# Summary

*   Removed unnecessary variables
*   Renamed variables that have same name as reserved word (`p`)
*   Other tiny fixes

I fixed message to make them more natural English.

Refactor RSpec for AnnotateModels (4) - AnnotateModels.get_schema_info (without custom options) (#735)

I refactored test cases of AnnotateModels without custom options.

I refactored RSpec file for another large scale refactor in another PR.

The scope of this pull request is to allow annotate_models to generate models documentation using YARD. This is the first step, I'll add more features later.

I moved `puts` from `AnnotateRoutes.rewrite_contents` to `.do_annotations` and `.remove_annotations`.

* Structuralize RSpec test cases of AnnotateModels.get_schema_info (with custom options)

* Replace expression expansion to plain text

* Refactor RSpec test cases of AnnotateModels.get_schema_info (with custom options)

* Change position of test cases

I removed `AnnotateRoutes.rewrite_contents_with_header` because the logic of `AnnotateRoutes.rewrite_contents_with_header` is as same as `.rewrite_contents`.

* Add columns managed by Globalize gem

Globalize hooks into the model and removes the translated columns
from the `klass.columns`. This commit checks if globalize is
hooked into the model and adds the necessary columns to the
annotation array.

* Disable Rubocop Metrics/BlockLength for spec files

RSpec spec files can contain long blocks easily because
of the outher describe methods. So this rule makes not too much
sense for these files.

I refactored `AnnotateRoutes.annotate_routes` and `.rewrite_contents_with_header` in order to add an empty string to `new_content` array in `.annotate_routes`.

I will refactor `.rewrite_contents_with_header` more, and remove `. rewrite_contents_with_header` in the next PR.

# Summary

*   I replaced `#blank?` of ActiveSupport to `#present?`
*   I moved `AnnotateModels::Parser#commit` after `#parser ` because `#commit` is used after `#parser` in `#parse`.

I removed invalid document of AnnotateRoutes.rewrite_contents.

I will add valid documents in YARD format in another PR.

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)