Updated installation for Rails 4

First time editing textile, any pointers are appreciated (and ways to view locally). This PR updates the installation instructions to add Rails 4. I tested on both 3.2.18 and 4.1.1. The frameworks listed in config/application.rb are from the corresponding new Rails repos (minus active_record/railstie).

https://github.com/mongomapper/mongomapper/issues/573
A bit of clarification that the linked resource is for upgrading from Rails 3 to 4 would probably be helpful. Maybe something more like `But, not everyone is starting fresh (Andy Lindeman has "an eBook about upgrading from Rails 3 to 4":https://github.com/alindeman/upgradingtorails4). If you're not upgrading, but just converting...`