make router required on context
make router required on context

we used to allow this to make unit tests easier, but now all you have
to do is wrap your component in a `<MemoryHistory>` and be on your way

closes #3889
DelvarWorld/react-router,OpenGov/react-router,goblortikus/react-router,asaf/react-router,reactjs/react-router,ReactTraining/react-router,OpenGov/react-router,ReactTraining/react-router,asaf/react-router,DelvarWorld/react-router,d-oliveros/react-router,goblortikus/react-router,goblortikus/react-router,rafrex/react-router,asaf/react-router,OpenGov/react-router,rackt/react-router,reactjs/react-router,d-oliveros/react-router,ReactTraining/react-router,goblortikus/react-router,rackt/react-router,rafrex/react-router,OpenGov/react-router,ReactTraining/react-router