Configure asset host in development mode

When run under `govuk_setenv`, this configures the asset host to point
at the assets-origin on the dev VM.  This will give us better dev-prod
parity, and enable running this behind the router in dev.
Merge pull request #125 from alphagov/asset-host-in-dev

Configure asset host in development mode
Replace travis with jenkins

This yields branch feedback much faster
Merge pull request #126 from alphagov/replace-travis-with-jenkins-branch-build

Replace travis with jenkins
Bump govuk_frontend_toolkit to 1.4.0

This brings the govuk_frontend_toolkit up to the latest version to
pick up lots of fixes and improvements.