Refs #4415 Enable HHVM build on Travis, I expect some tests will fail…

… so let's allow failures. Exciting...

.travis.yml

@@ -5,6 +5,7 @@ php:
   - 5.5
   - 5.4
   - 5.3
+  - hhvm
 
 # Separate different test suites
 env:
@@ -19,6 +20,8 @@ env:
     - TEST_SUITE=CoreTests MYSQL_ADAPTER=MYSQLI
     # Javascript tests
     - TEST_SUITE=JavascriptTests MYSQL_ADAPTER=PDO_MYSQL
+    allow_failures:
+      - php: hhvm
   global:
     - secure: "AMhZmPZx4SUcuZRBzGHlQPxzM4D8FvFB3UThDa52gbi9KIBrwcumzV2VGi6B\n5fgjwtB4XTE1In7qhY2HMikPWBmWYYOQ5QcMPJsqqHt4iMmahx8WKzne6NOk\nNpqAuje/fulNGeP2LJZi0nrub3Fh4VwXaOvpNloKNQN/2JuqPtM="
     - secure: "DySde80fX3dw0osGY5s5lGiGLjMleUkkONsuRDC4xjT1hAQP94FGHpPXVZV8\nAQY0s8MQmcK2eRLOQrmE9Bh1XXbGqh40s66XhSWq0hulFHxQH5OLaZXccFeu\n5WxGkHXnNDIc+CVhXW/IIbHZPO1K6hffS8fuvP1o6mYgxwFL6Xw="
@@ -30,6 +33,8 @@ matrix:
     env: TEST_SUITE=JavascriptTests MYSQL_ADAPTER=PDO_MYSQL
   - php: 5.5
     env: TEST_SUITE=JavascriptTests MYSQL_ADAPTER=PDO_MYSQL
+  - php: hhvm
+    env: TEST_SUITE=JavascriptTests MYSQL_ADAPTER=PDO_MYSQL
   # Mysqli tests need to run only on one PHP version
   - php: 5.4
     env: TEST_SUITE=IntegrationTests MYSQL_ADAPTER=MYSQLI