Remove greenkeeper postpublish hook

Author: bhousel

package.json

@@ -25,15 +25,13 @@
   },
   "devDependencies": {
     "eslint": "^4.0.0",
-    "greenkeeper-postpublish": "^1.0.1",
     "tap": "^10.1.0"
   },
   "engines": {
     "node": ">=4.0.0"
   },
   "scripts": {
     "lint": "eslint bin/spritezero test/",
-    "test": "npm run lint && tap test/test.js",
-    "postpublish": "greenkeeper-postpublish"
+    "test": "npm run lint && tap test/test.js"
   }
 }