Fix webpack versions while in beta

[griffinmichl]

Avoid breaking changes while webpack 2 is in beta.

[griffinmichl]

@phil303 👓

[phil303]

🐟 I'll handle getting this published to npm for now and I'll see about getting you npm publishing access. My publishing workflow is normally this:

1. Wait till my PR is fished.
2. Rebase down if I need to and double-check I'm publishing the new build stuff (since no repos have the prepublish step yet)
3. Rebase onto mainline branch
4. npm version <patch|minor|major>
5. git push upstream master (or whatever your set up is)
6. npm publish