Add npm 'prod' script to run 'gulp prod'

This adds a new 'scripts' entry to 'package.json' to run 'gulp prod',
eliminating the need for a global install of gulp.

Change-Id: I383e5bf99f6e9e2c7d5fa5c894ef573fa06facd7

package.json

@@ -63,6 +63,7 @@
   "scripts": {
     "postinstall": "if [ ! -d .venv ]; then tox -epy27 --notest; fi",
     "test": "gulp unit",
-    "lint": "eslint ./"
+    "lint": "eslint ./",
+    "prod": "gulp prod"
   }
 }