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/
Recently, we've needed to add some updates for the new releases. I believe they have been most ERs, but one or two deprecations came up. I would feel safer locking this down to avoid the "I updated and it broke" scenario. This way our support is explicit and Bundler / Rubygems will inform people of mismatches ahead of time. See history in #1187 and #1192.