Test against activesupport 4.0

Hi, this gem would be valuable for earlier versions of Rails (especially for an ActiveJob backport).  Added an additional Gemfile based on the structure of https://github.com/rails/protected_attributes.  All tests pass without any code changes, but looking at the `# TODO: may relax this dependency further` comment, it seems there may be some hesitation to do this.  Curious to hear your thoughts.  Thanks!