Make the vendor bundle hold vendor code

By setting the vendor libs as entry points, we force the vendor code
into the vendor-bundle.js. This has the advantage in that app code
should be changing less often, so client browsers will have the vendor
bundle cached longer, and the smaller app bundle can be refreshed.

Warning on what gets included:

If in your vendor array you have react-router and
somewhere in the codebase you import react-router/es6 — this import
won't get to vendor bundle.