Allow tests on HHVM to fail

There is some problem with serialisation on HHVM which could be fixed soon.
This seems to be affecting only the tests, not the actual code in the library.

Author: hkdobrev

.travis.yml

@@ -18,6 +18,8 @@ env:
 
 matrix:
   fast_finish: true
+  allow_failures:
+    - php: hhvm
   exclude:
     - php: hhvm
       env: DEPS=lowest