Update hack-codegen requirement to v2.1

Author: fredemmott

composer.json

@@ -5,7 +5,7 @@
     "homepage": "https://github.com/hhvm/hack-router-codegen",
     "require": {
         "facebook/hack-router": "^1.0.0rc7",
-        "facebook/hack-codegen": "^2.0.0",
+        "facebook/hack-codegen": "^2.1.0",
         "facebook/definition-finder": "^1.3.0",
         "fredemmott/type-assert": "^0.2.0 || ^1.0"
     },