Update rails2.rb self.use? to better detect Rails 2 apps

I originally wanted to use `gem_is_bundled?`, however every time I pushed to my app using this custom buildpack I was getting a "no Cedar-supported app detected" message, so I stole the detection method from `rails3.rb`. Not being too familiar with Rails 2 I'm not sure if the `Rails::Initializer ` is as reliable as I would like. 

Fixes heroku/heroku-buildpack-ruby#55