Add extra cheapseats entries to the travis build matrix

- Push travis's build matrix (http://docs.travis-ci.com/user/speeding-up-the-build/) to run 5 jobs concurrently
Revert "Pin nodejs to the version we use locally."

This reverts commit c3383199d0446a211b5b2f1f6abf385e3c47a407.

Travis only provides the latest stable version (0.10.x), so 0.10.31
is no longer available on some machines.
Merge pull request #833 from alphagov/more-pseudo-concurrency

Add extra cheapseats entries to the travis build matrix