fix: Downgrade babel-runtime to v5

This is needed because of https://phabricator.babeljs.io/T6644.

Author: dignifiedquire

package.json

@@ -26,7 +26,7 @@
     "babel-loader": "^6.2.0",
     "babel-plugin-transform-runtime": "^6.1.18",
     "babel-preset-es2015": "^6.0.15",
-    "babel-runtime": "^6.2.0",
+    "babel-runtime": "^5.8.34",
     "concurrently": "^1.0.0",
     "eslint-config-standard": "^4.4.0",
     "eslint-plugin-standard": "^1.3.1",