Make sure bundler is installed

Travis seems to have forgotten about installing bundler in our latest
builds, causing them to fail. We should probably just ensure it has
always been installed before we start trying to install gems. This
shouldn't affect the build if Travis decides to reinstate bundler.
Make rvm use 1.9.3

I've had a chat to the Travis guys in IRC and they suggested forcing rvm
into 1.9.3 to help fix this issue. They will be rolling out a fixed VM
later today.
Merge pull request #194 from alphagov/travis-is-annoying

Make sure bundler is installed