enable allow-plugins to fix composer install (#86)

Author: Scott Sandler

composer.json

@@ -24,5 +24,10 @@
     "hhvm/hhast": ">=v4.135.1",
     "facebook/fbexpect": "^2.8.0",
     "hhvm/hacktest": ">=2.2.3"
+  },
+  "config": {
+    "allow-plugins": {
+      "hhvm/hhvm-autoload": true
+    }
   }
 }