Finish Travis build as soon as a single job failed.

Manually cancelling obviously failing builds is getting cumbersome.
Hopefully this will speed things up as a side effect.

References:
- https://docs.travis-ci.com/user/customizing-the-build/#Fast-Finishing

.travis.yml

@@ -11,10 +11,11 @@ branches:
     - develop
 
 matrix:
+  fast_finish: true
+
   include:
 
     # 1/ Linux Clang Builds
-
     - os: linux
       compiler: clang
       addons: &clang38