Lock down rspec at a fully functional version

Rspec 3, which is currently installed by performing a `bundle install`, is incompatible with the version of spork installed by the same (see [stackoverflow](http://stackoverflow.com/questions/24030907/spork-0-9-2-and-rspec-3-0-0-uninitialized-constant-rspeccorecommandline-n)). Restricting rspec to less than 3, installs a compatible version.