Override React warning module to make an alert

Test Plan:
Followed instructions to build into webapp, changed a prop in the style guide to be invalid, then reloaded /style and got an alert. Switched to use the prod builds in javascript-packages.json and javascript/node_modules/react/index.js and didn't get an alert.

{F41384}

Reviewers: benkomalo

Reviewed By: benkomalo

Subscribers: joel, jessie

Differential Revision: http://phabricator.khanacademy.org/D11938