[RFC] Add script to manually trigger Travis builds.

Requires [jq](https://stedolan.github.io/jq/) and either a Github token in `GH_TOKEN` or a Travis access token in `TRAVIS_TOKEN`, e.g. `GH_TOKEN=... ./scripts/trigger-build.sh`.
I'm not sure how I feel about having my tokens appearing on the command line.  If this were written in something else, we could avoid the issue and only have the token in the environment.