ci(travis): enable fast finish mode for optional modes. (#10588)

* Travis CI runs the optional tasks in "waiting" mode, which means that the whole build is only ready once the optional tasks are also ready.
   e.g https://travis-ci.org/angular/angular/builds/150844963 - This build lost about 48 minutes just from an optional mode.

* The Travis Fast Finish mode will mark the build as finished, once all "required" tasks have finished.
   https://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

.travis.yml

@@ -47,6 +47,7 @@ env:
    - CI_MODE=browserstack_optional
 
 matrix:
+  fast_finish: true
   allow_failures:
     - env: "CI_MODE=saucelabs_optional"
     - env: "CI_MODE=browserstack_optional"