Enhancement: Allow to finish the builds as soon as possible

[localheinz]

This PR

• allows builds to finish as fast as possible

💁‍♂️ For reference, see https://docs.travis-ci.com/user/customizing-the-build#Fast-Finishing:

If some rows in the build matrix are allowed to fail, the build won’t be marked as finished until they have completed.

To set the build to finish as soon as possible, add fast_finish: true to the matrix section of your .travis.yml like this:

matrix:
 fast_finish: true

Now, a build will finish as soon as a job has failed, or when the only jobs left allow failures.

[localheinz]

Closing in favour of 6a7c407, see #721.