update for Babel 6

Instead of just downloading this repo (which works great), I tried following your [blog post](http://www.dwmkerr.com/getting-started-with-react/) in order to learn the process better. However in the blog, you don't specify Babel 5.x (even though you do in the package.json in this repo), so I ended up with a broken stack because I just installed the latest Babel (6+).

Support Babel@6 is a pretty simple change: https://medium.com/@malyw/how-to-update-babel-5-x-6-x-d828c230ec53#.7xxg7di94. If you're willing to accept it, I can create a pull request.

Start with the blog post and package.json, then use the linked Babel 5-to-6 migration guide to identify the starter's compatibility changes. Verify the documented setup no longer installs a broken stack when Babel 6 is current, and confirm the repository still downloads and runs as described.

Written by the indexing model from the issue text.