Add HHVM to Travis

Currently 1 test is still failing, so it's set up to allow for failures.
See: https://github.com/facebook/hhvm/issues/2018

Signed-off-by: Jeff Chan <jefftchan@gmail.com>

.travis.yml

@@ -3,6 +3,7 @@ php:
   - "5.5"
   - "5.4"
   - "5.3"
+  - hhvm
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get install -qq ant
@@ -19,3 +20,7 @@ script:
   - if [[ $TRAVIS_PULL_REQUEST != "false" ]] ; then ./scripts/check-dco origin/$TRAVIS_BRANCH..FETCH_HEAD ; fi
 after_script:
   - php vendor/bin/coveralls -v
+matrix:
+  allow_failures:
+    - php: hhvm
+  fast_finish: true