(chore): Only test Rails 5 against Ruby 2.4.0-pre
(chore): Only test Rails 5 against Ruby 2.4.0-pre

This gives us some visibility of the upcoming version, but without
having to deal with the fact that rails 4.1 and 4.2 don't seem to work
with it ("stack level too deep" in activesupport).

Added bonus: don't run too many builds on Travis

dsandstrom/foundation_rails_helper,sgruhier/foundation_rails_helper