[fixed] React dependency

It should be in peerDependencies, not devDependencies.  Otherwise,
you'll get needless failures on `npm list`.

Note: if you are developing ReactRouter itself, your repo should be
inside your calling app's node_modules until this is fixed:

https://github.com/npm/npm/issues/5875

If you try to instead resolve this by installing a copy of React inside
ReactRouter's node_modules, you're liable to end up with a broken
build, because webpack will include both copies in parallel.