Update to lerna 4

Author: jtpio

package.json

@@ -24,7 +24,7 @@
   "devDependencies": {
     "@jupyterlab/buildutils": "^3.0.0",
     "husky": "^0.14.3",
-    "lerna": "^3.4.1",
+    "lerna": "^4.0.0",
     "lint-staged": "^7.1.0",
     "prettier": "^1.11.1",
     "rimraf": "^3.0.2",