Add HHVM to build matrix

.travis.yml

@@ -5,6 +5,7 @@ php:
   - 5.3
   - 5.4
   - 5.5
+  - hhvm
 
 env:
   - TWIG_EXT=no
@@ -13,3 +14,8 @@ env:
 before_script:
   - if [ "$TWIG_EXT" == "yes" ]; then sh -c "cd ext/twig && phpize && ./configure --enable-twig && make && sudo make install"; fi
   - if [ "$TWIG_EXT" == "yes" ]; then echo "extension=twig.so" >> `php --ini | grep "Loaded Configuration" | sed -e "s|.*:\s*||"`; fi
+
+matrix:
+  exclude:
+    - php: hhvm
+      env: TWIG_EXT=yes