Start testing hhvm on travis-ci

Lets see if it even has imap enabled :-)

.travis.yml

@@ -4,6 +4,7 @@ php:
     - 5.3
     - 5.4
     - 5.5
+    - hhvm
 
 before_script:
     - composer self-update && composer install --dev
@@ -14,3 +15,7 @@ script: ./tests/runTests.sh
 
 after_script:
     - php vendor/bin/coveralls -v
+
+matrix:
+    allow_failures:
+      - php: hhvm