[travis] Allow hhvm and nightly to fail

.travis.yml

@@ -10,4 +10,9 @@ install:
 
 script:
   - phpenv rehash
-  - phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p
+  - phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p
+
+matrix:
+  allow_failures:
+    - php: hhvm
+    - php: nightly