Add HHVM to the build matrix of Travis-CI for v0.8.

Travis-CI recently provisioned virtual machines with HHVM 2.4.0 which
fixed the issues preventing Predis v0.8 from working correctly.

.travis.yml

@@ -3,6 +3,7 @@ php:
   - 5.3
   - 5.4
   - 5.5
+  - hhvm
 branches:
   except:
     - v0.5
@@ -11,3 +12,7 @@ branches:
     - documentation
 services: redis-server
 script: phpunit -c phpunit.xml.travisci
+matrix:
+  allow_failures:
+    - php: hhvm
+  fast_finish: true