Add Libffi-dev installation to the bootstrapping script

[peterb]

Currently when you run ./railties/exe/rails new cool_app --dev inside the Vagrant container, you get a message: along the lines of Error installing ffi: Failed to build gem native extension

If libffi-dev is installed, the libffi gem native extensions can compile, which are required when generating a Rails application inside the Vagrant container. See my post on the rails-core mailing list for some background on the motivation, this is one approach. Happy to modify/add to this this based on feedback!