Add babel-runtime to the default new-app package.json file.

In light of issues like #7956, I would very much like for app developers
to be responsible for providing node_modules/babel-runtime, and for the
Meteor babel-runtime package to stop attempting to implement a subset of
the helpers.

tools/static-assets/skel/package.json

@@ -5,6 +5,7 @@
     "start": "meteor run"
   },
   "dependencies": {
-    "meteor-node-stubs": "~0.2.0"
+    "meteor-node-stubs": "~0.2.0",
+    "babel-runtime": "6.18.0"
   }
 }