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.

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>

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 

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.

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.

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>

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>

* Update rubocop requirement from ~> 0.46.0 to ~> 0.67.2

Updates the requirements on [rubocop](https://github.com/rubocop-hq/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop-hq/rubocop/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop-hq/rubocop/compare/v0.46.0...v0.67.2)
Signed-off-by: dependabot[bot] <support@dependabot.com>

* Make changes to let Rubocop pass in CI

* Fixed some offenses
* Added test.

Fixed some offenses

Reduced the Cyclomatic complexity.
#330 By default didn't annotate activeadmin models.
This close #330

This conversion is done by Transpec 2.2.5 with the following command:
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* 27 conversions
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* 18 conversions
    from: == expected
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* 4 conversions
    from: obj.should_not
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* 4 conversions
    from: obj.stub(:message)
      to: allow(obj).to receive(:message)

For more details: https://github.com/yujinakayama/transpec#supported-conversions