Drop code related to Rails < 5.0 & Ruby < 2.3.0

[pirj]

Another cleanup attempt (former: #2225).

Note: in the Changelog, we state that we drop support for Ruby versions below 2.3, even though Rails 5.2 supports Ruby 2.2.2. It's a matter of getting just a couple lines back to keep 2.2.2 support.

I didn't dare to refactor FixtureFileUploadSupport.

[pirj]

@JonRowe It's a big one, but mostly removals of old Rubies/Rails 3/4 support. Appreciate if you glance over.

[benoittgt]

For Rails 4.2 did you read this comment #2071 (comment)
I am wondering how much we should clean code related to Rails 4.2

Thanks for the work.

[pirj]

@benoittgt Thanks for pointing up, I completely missed the second part of this comment.
I'll check again if there's something pre-4.2 that can be dropped.

[JonRowe]

Pretty much all of this was good to go @pirj I'd reopen it as is mostly.

At 4.0 release time, 4.2 will be removed from the build matrix, but not the gem spec

This means we can remove all of our spec / feature tests for Rails 4.2

4.2 will not be officially supported in RSpec Rails 4, and bugs against that version will not be fixed

This means that we should leave the checks for 4.2 methods to check they are supported before calling in lib, and the same for files. However anything that is old Ruby is ok to be dropped. We're hard supporting 5.0 and soft supporting 4.2 but not old Ruby, thats gone.

A warning will be issued at require time, if a user is using Rails 4.2, indicating to them that while the version is unlikely to be supported, bug fixes will not be issued against 4.2

This needs to be added somewhere.

[benoittgt]

Sorry @pirj I was maybe not clear enough. But I don't this PR should be closed. :)

[JonRowe]

@pirj let me know when this is ready to review again :)

[pirj]

@JonRowe @benoittgt Please take another look.

I broke four examples on 4.2, mailer_generator_spec.rb and configuration_spec.rb, by removing < 5.0 checks in those specs. The code should work fine. If you comment them out

RAILS_VERSION=4.2.11 rake

expectedly get stuck at the point where @rails_post_5 tags were.

[JonRowe]

It looks good but I want to hold off final review until we've got green CI

[pirj]

@JonRowe @benoittgt Green!

[JonRowe]

One change really, only due to our process, currently the dev build should apply cleanly to rspec-rails and I'd like to keep it that way for centrally managed files

[pirj]

All green. Reverted changes to predicate functions. Squashed commits.