Use Rails test runner for Ruby on Rails projects

[amake]

I use ruby-test-mode with some Ruby on Rails projects and I find that the experience using the Rails test runner is superior to other runners. For instance

• The other runners fail for some of my projects due to missing imports, while rails test works; this may be a configuration issue on my end, but it's hard to figure out
• The -v flag with the Rails runner gives nice output that gives me a better idea of the progress of a long-running test suite

This PR adds the capability to use rails test for Rails projects.

[amake]

The Emacs 26.3 CI failure seems unrelated to the content of the PR (it's failing to provision cask).

[icostan]

Yeah, that's true, something is wrong with cask and emacs-26 version, I am going to fix it this weekend. Merging the PR for now.

[rtlong]

I think this has broken my setup, where I use rspec with a Rails project. I normally do not use the rails test runner, instead I use the rspec executable directly. When I saw this change, I looked, hard, to find any docs on how to use the rails-test runner with Rspec, and either it's not supported or can't find how to do it. I don't see anywhere in the Rails source where that command would at some point defer to RSpec's logic. To work around this, I'm going to advise the ruby-test-rails-p func to return false. I'd love to understand how other folks are getting on with a similar setup; maybe it's just a configuration in Rails I'm missing or maybe this change is problematic for that workflow?

[amake]

@rtlong Sorry for the breakage. I thought the Rails runner would be smart, but I guess it's not.

Please see if #65 fixes it for you.