Fix Travis build by pinning Rspec at 2.14.x and switching to rbx-2

The capistrano Gemfile did not specify a version for Rspec, so when the new version 3 of Rspec was released, Travis began using that latest version. Unfortunately capistrano's tests don't pass with Rspec 3, so I've updated the Gemfile to pin Rspec at 2.14.x.

Another reason Travis was failing was due to rvm not being able to install rbx. This Stack Overflow thread suggested using rbx-2 instead, which seems to have fixed the problem.

http://stackoverflow.com/questions/23586309/rbx-tests-fail-with-requested-binary-installation-but-no-rubies-are-available-t