Updates to allow use with Rails 4

1. Loosened gem restrictions in gemspec
2. Added a `require 'yaml'` to the Obscenity::Base class
3. Added Ruby 2.1.0 to the .travis.yml
4. Moved method definitions in TestRack out of the context block, so the spec runs.
5. Add Gemfile.lock to .gitignore and remove Gemfile.lock from the repo, so we can easily run against multiple VMs.