fix(package): add missing greenkeeper-postpublish script

Author: Hans Kristian Flaatten

package.json

@@ -16,7 +16,8 @@
     "nsp": "nsp check",
     "semantic-release": "semantic-release",
     "test": "mocha -r test/support/env -R tap -b --check-leaks test/**",
-    "test:watch": "mocha -r test/support/env -R progress -b --check-leaks -w test/**"
+    "test:watch": "mocha -r test/support/env -R progress -b --check-leaks -w test/**",
+    "greenkeeper-postpublish": "greenkeeper-postpublish"
   },
   "repository": {
     "type": "git",