Cache vendor/{bundle,modules} on travis
Cache vendor/{bundle,modules} on travis

So that each run doesn't have to install all our gems and external
modules again.
This will speed up travis runs by about 50 seconds for the bundle
install at least.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet