Ensure Travis runs tests
Ensure Travis runs tests

Move tests into their own build stage. It appears the script to run the
tests is being ignored without this.

This can be seen working here:
https://travis-ci.org/alphagov/govuk_elements/builds/234778197#L217

and then no longer working here:
https://travis-ci.org/alphagov/govuk_elements/builds/246248025

alphagov/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements