Split grunt test tasks to make unit tests a separate task

So that the unit tests a.k.a the "fast" tests can be run separately from the full test suite, which is slow.
Update cheapseats version to include range parameters
Split travis build into 3 parts

One which runs the unit tests, and two which run (roughly) half of cheapseats each.
These can then be parallelised across multiple VMs so should sped up the process.

See http://docs.travis-ci.com/user/speeding-up-the-build/ for details.
Merge pull request #520 from alphagov/feature/optimise-travis

Feature/optimise travis