[Travis] enable fast_finish

With "fast_finish" enabled, Travis CI will mark builds as finished
as soon as a job has already failed (or the only remaining ones are
allowed to fail).

https://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

.travis.yml

@@ -3,6 +3,7 @@ sudo: false
 language: python
 
 matrix:
+  fast_finish: true
   include:
     - python: 2.7
       env: TOXENV=py27-cov