Update gem to point to demo API URL.

- Update gems in Gemfile to match octokit.rb

- Update tests and cassettes to point to the demo API

- Remove unused keywords module and tests

- Update client#paginate method

- Update rate_limit module to take into account case where rate limit headers are not present

- Update categories module to be compatible with demo API

- Add per_page to configurable and default modules

- Update README examples to work with demo API