Use published extension builder

jupyterlab/package.json

@@ -10,6 +10,7 @@
     "font-awesome": "^4.6.1",
     "fs-extra": "^0.30.0",
     "jupyterlab": "file:../",
+    "jupyterlab-extension-builder": "^0.1.0",
     "material-design-icons": "^2.2.3",
     "semver": "^5.3.0",
     "webpack-config": "^6.1.2"