bump hacktest dependency to 2.0

Author: jjergus

composer.json

@@ -18,7 +18,7 @@
     "facebook/fbexpect": "^2.0",
     "hhvm/hhast": "^4.0",
     "hhvm/hhvm-autoload": "^2.0.8",
-    "hhvm/hacktest": "^1.1",
+    "hhvm/hacktest": "^2.0",
     "usox/hackttp": "^0.3"
   }
 }