Add hhvm.

Signed-off-by: crynobone <[email protected]>

Author: crynobone

Diff for: .travis.yml

@@ -4,6 +4,7 @@ php:
   - 5.3
   - 5.4
   - 5.5
+  - hhvm
 
 before_script:
   - composer self-update
@@ -13,3 +14,7 @@ script: phpunit -c phpunit.xml --coverage-text
 
 after_script:
   - php vendor/bin/coveralls -v
+
+matrix:
+  allow_failures:
+    - php: hhvm