Fix build error

Author: jakezatecky

package.json

@@ -16,6 +16,7 @@
   "bugs": "https://github.com/jakezatecky/react-checkbox-tree/issues",
   "main": "lib/index.js",
   "scripts": {
+    "build": "gulp build",
     "examples": "gulp examples",
     "gh-deploy": "git subtree push --prefix examples/dist origin gh-pages"
   },