Add libffi and libffi-dev, needed for building Ruby 2.2.0

I had trouble compiling Ruby 2.2.0 until I installed libffi and libffi-dev, as suggested here: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/67129

This PR adds libffi and libffi-dev to the Ubuntu 12.04 bootstrap.