Upgrade webpack.

Summary:
I previously got this:

> npm ERR! peerinvalid The package webpack does not satisfy its siblings'
> peerDependencies requirements!
> npm ERR! peerinvalid Peer webpack-dev-server@1.4.7 wants webpack@1.x

The newest versions seem to work.

Test Plan: `make server` succeeds, the test page looks good at first glance

Reviewers: aria

Differential Revision: http://phabricator.khanacademy.org/D13519