Set Content-Type for live.bundle.js and webpack-dev-server.js

Browsers try to auto-detect files without a Content-Type, and warn in many cases. Google Chrome for example detects the files as text/plain, and then warns when they are used as scripts: 'Resource interpreted as Script but transferred with MIME type text/plain: "http://localhost:8080/__webpack_dev_server__/live.bundle.js".'