Add HHVM to the build matrix on Travis CI.

We allow failures on builds run against HHVM since it is still being
developed and things could break anytime, but at least this gives us
a decent indicator of the level of compatibility of Predis with this
runtime.

.travis.yml

@@ -3,6 +3,10 @@ php:
   - 5.3
   - 5.4
   - 5.5
+  - hhvm
+matrix:
+  allow_failures:
+    - php: hhvm
 branches:
   except:
     - v0.5