Add travis_retry to Travis build process

Do it for things that use the network to install things.

This blog post is from May 2013 but we're still seeing issues in March
2014 and it's starting to get annoying enough that I'm considering
running branch builds on our own CI stuff instead.

http://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries/
Merge pull request #292 from alphagov/bugfix-yo-travis-please-retry-things

Add travis_retry to Travis build process