Recommend installing 'ruby-dev' on Ubuntu

sudo gem install ffi -v '1.9.3'

...was failing

http://stackoverflow.com/questions/18316667/failed-to-build-gem-native-extension-mkmf-loaderror-ubuntu-12-04/18316692#18316692
Merge pull request #2835 from guardian/require-ruby-dev-on-ubuntu

Recommend installing 'ruby-dev' on Ubuntu