Use babel 6 for now

Author: Timer

packages/react-error-overlay/package.json

@@ -33,7 +33,7 @@
   "devDependencies": {
     "anser": "1.4.1",
     "babel-code-frame": "6.22.0",
-    "babel-core": "^7.0.0-beta.2",
+    "babel-core": "^6.26.0",
     "babel-eslint": "7.2.3",
     "babel-loader": "^7.1.2",
     "babel-preset-react-app": "^3.0.3",