Upgrade react-router from v2 to v3
Upgrade react-router from v2 to v3

Summary:
This is necessary to fix the deprecation warning for `React.PropTypes`.
There don't appear to be any breaking changes in my use of the API.

Test Plan:
Navigate around the site, then refresh some pages and make sure that
they still load correctly. Then, do the same with `npm run serve`.

wchargin/wchargin.github.io,wchargin/wchargin.github.io