Updated Gemfile, Rakefile

When trying to use another gem that also required the Heroku gem, I found out that I couldn't use workless anymore. I went through and updated the Gemfile to have a better set of gems, including moving shoulda to the development and test group.

I've tested it out by running on Heroku with this new gem and it works fine. I didn't update the version number of the gem, since I figured you would do that.