Set Rails 5.2.0 in templates

Prepare bug report templates for Rails 6.0 development

Add missing `require "active_support"` in `guides/bug_report_templates/generic_gem.rb`

[ci skip]

:github source uses `git://` url by default, `https://` is recommended.
See http://bundler.io/v1.15/guides/git.html#security

We do the same in our `Gemfile` and templates.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

5.1.0 has been released, and the gem templates can reflect that now.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

The bug report templates are now executed from the `ci/travis.rb` when
`GEM` contains `guides`.

I started by creating a `test` task in `guides/Rakefile` to handle this,
but since inline `gemfile` must not be executed with `bundle exec`, that
rake task would not be consistent with others. So I went back by
executing them directly from `Build`.

Use inline Gemfile dependency when reporting gem bugs

 - Generally we have to run the bug templates multiple times to get them
   right and it always complains because the posts and comments tables
   already exist due to earlier runs.
 - Using force: true will eliminate this issue.

- Right now master is 5.0.0. Latest gem release is 4.2.0 for which we
  are accepting bug reports. So lets use it in bug report templates.
- 5.0.0 is not installable as it's not available on Rubygems yet. So the
  gem bug templates are not usable without editing the version. Using
  4.2.0 will make them usable again.

Define Minitest::Test in case we are running the gist under Minitest 4
and avoid relying on MiniTest::Unit::TestCase to avoid displaying
warning on version five.

[ci skip]

Ref: https://github.com/seattlerb/minitest/blob/master/History.txt

Since Rails 4.0.x is depending on Minitest 4.x, the constant won't be
available so the gist won't run.

[ci skip]

* use `Minitest::Test` instead of deprectaed `MiniTest::Unit::TestCase`

 [ci skip]

since users just copy and paste and the class name is never changed

This commit closes issue #9775.