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/
If say a 4.3 does come out and we don't need code changes, I see no problems in releasing patch dot release bumping the gem support.