Run librarian-puppet install on travis.
Run librarian-puppet install on travis.

Without this, none of the vendored modules are available, which leads to
lots of things breaking.

boffbowsh/ci-puppet,alphagov/ci-puppet,alphagov/ci-puppet,boffbowsh/ci-puppet,alphagov/ci-puppet,boffbowsh/ci-puppet,boffbowsh/ci-puppet,alphagov/ci-puppet