Remove jsdom from test harness

I've had nothing but pain and tears from jsdom, the most recent of which happened when I did a fresh clone, `npm install` and `npm test`.

```
$ npm test

> redux-router@1.0.0-beta3 test /Users/michael/Projects/redux-router
> mocha --compilers js:babel/register --recursive --require src/__tests__/init.js src/**/*-test.js

1) "before all" hook

reduxRouter()
✓ adds router state to Redux store
✓ detects external router state changes (47ms)
✓ works with navigation action creators
✓ doesn't interfere with other actions
getRoutes()
✓ is passed dispatch and getState
onEnter hook
✓ can perform redirects
isActive
✓ creates a selector for whether a pathname/query pair is active

7 passing (137ms)
1 failing

1) "before all" hook:
/Users/michael/Projects/redux-router/node_modules/jsdom/lib/jsdom.js:3
`jsdom 4.x onward only works on io.js, not Node.js™: https://github.com/tmpvar
^
Unexpected token ILLEGAL
node_modules/jsdom/lib/jsdom.js:3
`jsdom 4.x onward only works on io.js, not Node.js™: https://github.com/tmpvar
^
SyntaxError: Unexpected token ILLEGAL
at exports.runInThisContext (vm.js:73:16)
at Object.require.extensions.(anonymous function) [as .js] (node_modules/babel/node_modules/babel-core/lib/api/register/node.js:214:7)
at Context. (node_modules/mocha-jsdom/index.js:52:5)
```

Can we remove it? I can help you guys get tests running in real browsers using a similar karma + webpack + mocha setup to what we're using in the router.