Congratulations!
Some quick feedback, you should add a `rspec-matchers-controller_filters.rb` file to your lib folder that just requires `rspec/matchers/controller_filters` so that your gem can be loaded by bundler automatically. You should also target `rspec-rails ~> 3.0` rather than 3 to ensure any minor version of RSpec 3, but without RSpec 4 (which could break your gem in future).
As an asidel you’re not really testing execution of action filters, but ensuring they’re setup, generally I’d expect this to be tested with integration tests rather than stubbing against the controller but if it works for you then great!
You could also setup TravisCI to ensure PR’s and such run your tests to help people contributing to your project.