Remove single child restriction for <Router>

[pshrmn]

This PR takes advantage of the fact that with React 16, a component can return multiple elements.

// 15
ReactDOM.render((
  <BrowserRouter>
    <div>
      <One />
      <Two />
    </div>
  </BrowserRouter>
), holder);

// 16
ReactDOM.render((
  <BrowserRouter>
    <One />
    <Two />
  </BrowserRouter>
), holder);

The invariant call will still throw in React 15 if the user passes multiple children to a <Router>. I'm not actually sure if there is a convenient way to test this. If someone is using React 15 and attempts to pass multiple children, the invariant will throw before the render function is called, so the changes to the render function should have no effect

[timdorr]

Is this possible at the top level? Didn't know that!

I'm not sure how I feel about a version check. Feels like browser sniffing, which is bad. It should at least be isolated to utility module so we can re-use it elsewhere and test it.

[pshrmn]

Yeah, I hadn't even thought about this until I was reminded of the "aux component" earlier

const Aux = props => props.children

I didn't want to do a version check, but I wasn't really sure how to continue supporting v15 otherwise.

I'm not actually sure if this is necessary. I preferred grouping the actual app components in a single <App>, but it would still be nice to get rid of the wrapper element.

[pshrmn]

I updated this to throw if multiple children are provided and React.Fragment does not exist. This isn't perfect since React.Fragment was added in 16.2; maybe this PR should just wait until RR bumps its React peer dep to >=16?