Make sure ruby version is consistent with rails version

This page instructs students to check to verify they've installed Rails 4, but then instructs them to check to see that they've installed Ruby 1.9.3. Rails 4 requires Ruby 2. 

I don't think it's super important that we decide on Rails 4/ Ruby 2 vs Rails 3 / Ruby 1.9.3, but it is important that these things are consistent. This prevents students from getting confused about conflicting versions.