Add integration test that calls on rails generator.

This PR does the following:
* moves code coverage generation to happen when `COVERAGE` environment variable is present.
* makes integration tests run when when `INTEGRATION_TESTS` is present.
* moves `ruby-head` from TravisCI build. Previously the configuration allowed for `ruby-head` to be ignored if it failed, but it conflicted with the gem release build job step.

In https://travis-ci.org/github/ctran/annotate_models/builds/659982898/config?utm_medium=notification&utm_source=github_status it looks like `travis.yml` is not valid and seems the `matrix` portion seems to be overwriting `jobs`, which is used to release the gem. 

This config is valid. I used `travis lint .travis.yml` to validate locally.

For further refactoring toward better architecture, I moved methods `.strip_annotations`, `.extract_magic_comments_from_array` and `.real_content_and_header_position` from `AnnotateRoutes` to `AnnotateRoutes::Helpers`.

In #764, it was pointed out that the gemspec requires ruby version `>= 2.2.0`. We've stopped testing for older versions of Ruby in CI and so it makes sense to make it explicit.

It was reported in https://github.com/ctran/annotate_models/issues/762 that column defaults were broken. This reverts changes made in #677 to restore the expected behavior of column defaults. 

For the time being columns with associated enums won't be working.

Bumps [puma](https://github.com/puma/puma) from 4.3.1 to 4.3.3.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](https://github.com/puma/puma/compare/v4.3.1...v4.3.3)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [puma](https://github.com/puma/puma) from 3.12.2 to 4.3.3.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](https://github.com/puma/puma/compare/v3.12.2...v4.3.3)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.10.7 to 1.10.8.
- [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/v1.10.7...v1.10.8)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.10.7 to 1.10.8.
- [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/v1.10.7...v1.10.8)
Signed-off-by: dependabot[bot] <support@github.com>

This is the final completed version of refactoring AnnotateRoutes.

* Fix new lines after comments for rubocop compatability

Fix #552, #607 

The test cases of AnnotateModels.get_schema_info was separated into two sections.
So I merged them into one, and refactor test cases.

This PR is new version of #741.

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.