respond_to? doesn't return true for dynamic methods added by method_missing

In both acts_as_follower and acts_as followable method_missing is overridden to add dynamic methods to the class.  But respond_to? returns false when called with the symbols for these dynamic methods.

This pull request update respond_to? to behave correctly.

Please note that this pull request also contains a gemspec update required to get the tests to run green.  This gemspec change was designed to just get the tests running, and it should be replaced by the gemspec changes in [this pull request](https://github.com/tcocca/acts_as_follower/pull/25) if they are both pulled in.

Thanks.