Drop dependency on mocha-jsdom in favor of explicit solution

[gaearon]

Instead of doing something mysterious with mocha-jsdom we now do as suggested in facebook/react#5046 (comment).

[zpratt]

@gaearon: I have been using a similar solution to this for my tests for quite some time. Recently we have switched to using the shallow renderer, which has allowed us to remove jsdom from our tests completely. This has given our tests a significant speed boost. Shallow rendering also forces you to keep each component relatively simple since it prevents you from testing heavily nested components.

Just thought I'd share a tip on something that we've found to be valuable.

[gaearon]

We are using shallow rendering too, take a closer look :-) The only reason we use jsdom is to prevent React from blowing up with errors when using setState with shallow renderer. (facebook/react#4019) I have heard people say that actually shimming global.document = {} is enough for React to work properly. If that's really so, we can drop jsdom from instructions.