Add license to package.json

This project includes the license text in the LICENSE file, but it doesn't indicate which license it uses in the package.json metadata, which doesn't play nicely with some tooling.

package.json

@@ -9,6 +9,7 @@
       "email": "ken.perkins@rackspace.com"
     }
   ],
+  "license": "MIT",
   "repository": {
     "type": "git",
     "url": "http://github.com/mmalecki/hock.git"