Update gemspec capping Rails at 4.2

According to the Rails blog the 4.2 release is the last in the 4.x line.
Plans for Rails 5.x support are still in the works and may not be on
rspec-rails 3.x.

To inform users and prevent accidental usage this limits the allowed
Rails versions in the gemspec.

- http://weblog.rubyonrails.org/2014/12/19/Rails-4-2-final/
While the rails team has publicly announced that 4.2 will be the last 4.x release, wouldn't it be slightly safer to do `< 5.0`?  If for some reason they decided to release 4.3 that would allow that version.  Unlikely, I know, but if your intention here is to explicitly not support rails 5+ that seems more semantically correct.  (OTOH, if you explicitly would not want to support 4.3 if that was ever released, this is fine).