Added Travis testing with HHVM

.travis.yml

@@ -1,15 +1,20 @@
 language: php
 
 php:
-    - 5.3
-    - 5.4
-    - 5.5
+  - 5.3
+  - 5.4
+  - 5.5
+  - hhvm
+
+matrix:
+  allow_failures:
+    - php: hhvm
 
 before_script:
-    - curl -s https://getcomposer.org/installer | php
-    - php ./composer.phar install --dev --prefer-source
+  - curl -s https://getcomposer.org/installer | php
+  - php ./composer.phar install --dev --prefer-source
 
 script: phpunit
 
 notifications:
-    email: false
+  email: false