[experimental] Drop support of Ruby < 2.2 & Rails < 5.2

[pirj]

As per Rails' support policy, Rails dropped all support for 5.0/5.1, even for severe security issues.

This comment mentioned that we planned to keep support for 5.0.
Another comment mentioning plans to drop 4.2 support.

From Versioning Strategy RFC:

Release a major with any Rails Major, removing support for any newly unsupported versions of Rails, and adding support for the new major.

This pull request has some rough edges, but I can iron it out if you feel like dropping support for < 5.2 is the way to go.

[pirj]

The only failure is YARD on JRuby mistakenly takes {lib} in

       gem lib, :git => "https://github.com/rspec/#{lib}.git", :branch => 'master'

for a reference. Appreciate any guidance here.

I'm getting a weird nokogiri issue locally, running rake fails to find 1.8.5. Update to nokogiri 1.9.1 (that requires Ruby 2.1+) fixes the issue.

PS rake passed locally.

[pirj]

Questions/TODO:

Is it possible to get rid of this?

# We need to copy this method from Thor for older Rails versions
def comment_lines(path, flag, *args)

Evaluate numerous respond_to? calls, e.g.:

          ::ActiveRecord::Migration.respond_to?(:check_pending!)

I18n.enforce_available_locales = true if I18n.respond_to?(:enforce_available_locales)

Simplify FixtureFileUploadSupport that has some trick to support Rails 3.

[JonRowe]

I seem to recall the original discussion was to drop support for 4.x in 4, but not 5.x, leaving that for later, I know thats not what the RFC says, but basically kicking out all of the previously supported rails versions at once is a bit much.

[JonRowe]

See #2071 for reference