Use matrix.fast_finish for the travis-ci to mark the build ASAP

AFAIR there is shortage of osx workers on travis-ci, IOW builds that
requires them can wait fair amount of time in the queue by just waiting.

Plus linux workers AFAICS can run multiple jobs in parallel (4-5), while
osx does not.

Hence if we do allow failures for osx (and right now they have a lot of
failed tests) let's mark build result based on tests under linux only.

So in a nut shell this will reduce build time from 5 hours to 20-30
minutes.

.travis.yml

@@ -27,6 +27,7 @@ matrix:
       env: EVENT_BUILD_METHOD=cmake EVENT_CMAKE_OPTIONS="-DEVENT__COVERAGE=ON -DCMAKE_BUILD_TYPE=debug" COVERALLS=yes
   allow_failures:
     - os: osx
+  fast_finish: true
 
 language: c
 compiler: