[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`.
This won't be printed out, just passed on to curl. A script run looks like:

```
TRAVIS_TOKEN not set; using GH_TOKEN for authentication.
Restarting latest build on fwalch/bot-ci.
The build was successfully restarted.
```