Rails 4, JRuby Support

This PR extends the Travis test matrix to include Rails 4 and JRuby.  Specific changes include:

1. Adding Appraisal to support test runs against different Rails versions
2. Adding specific Gemfiles for each of the active minor branches of Rails (3.0, 3.1, 3.2, 4.0)
3. Updating the .travis.yml file to use these new gemfiles, test against jruby-19
4. Pulling sqlite out of the gemspec, so we can test against JRuby
5. Restrict mass assignment tests to Rails 3.x
6. Update the gemspec to allow ActiveRecord 4.0.0

All specs run green on Travis.