Update Readme instructions for Test::Unit and Rails

The readme makes it look like that if you're using Test::Unit (or Minitest::Spec) with Rails, there's no need to add the teardown method given in the non-Rails snippet. However if you don't, the session and driver state will be retained between tests, which is different to the behaviour that Rspec gets by default. This PR attempts to reduce confusion that may arise (which is what happened in my case).

Ideally, I think Test::Unit / Minitest should get this behaviour by default just like Rspec does, perhaps through the [`after_teardown`](http://docs.seattlerb.org/minitest/Minitest/Test/LifecycleHooks.html#method-i-after_teardown) callback Minitest provides?