Report Travis CI build success early

This will report the build has succceeded even if allowed to fail jobs are
still running.

See https://github.com/composer/composer/pull/3828.

Docs: http://docs.travis-ci.com/user/build-configuration/#Fast-finishing

Props to @fonsecas72.

.travis.yml

@@ -21,6 +21,7 @@ php:
   - hhvm-nightly
 
 matrix:
+  fast_finish: true
   allow_failures:
     - php: hhvm-nightly