Support Rails 4.2, bump rspec version and fix specs

Hi, I bumped the Rails dependency (because Rails master is now working on 5.0, `< 5.0` should be safe) to use it with Rails 4.2 and upcoming minor releases. I also bumped the rspec minor version and fixed some deprecation warnings (replaced `stub` with `double` and added the `rspec-collection_matchers` gem).

Merry Christmas!