Make Travis deployment config consistent
Make Travis deployment config consistent

With other GDS FE repos, see:
https://github.com/alphagov/govuk_frontend_toolkit/pull/385

Steps specified in before_deploy are only run if a deployment is
triggered [1], which will only happen on master, which can't be
done by a user without write perms.

[1] [Travis before_deploy documentation](https://docs.travis-ci.com/user/deployment/npm/#Running-commands-before-and-after-deploy)

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