Expanded Travis tests

This PR updates the Travis configuration so the gem is tested against a full matrix of Ruby MRI and Rails versions.  Specifically:

1) Updated Gemfile to support a configurable Rails version.
2) Updated .travis.yml to use this environment variable.
3) Removed the Gemfile.lock and added it to the .gitignore
4) Added a default Rake task to run the specs.
5) Fixed a bug in Rails 3.0.0 where an additional argument was being passed to update_attributes.