Don't lock rspec-rails to old version

https://gemnasium.com/crowbar/travis-ci-crowbar revealed that
rspec-rails was locked at 2.11.4 while the latest version is 2.13.0.
There doesn't seem to be any reason to lock it at the old version, so
removing the constraint.

Tested on both Ruby 1.8.7 and Ruby 1.9.3.