[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`.
On Python 3, it fails with `Error: the JSON object must be str, not 'bytes'`. If I understood this correctly, Python 2 strings are just bytes, so it's no problem there.