Require Duplex earlier

I'm seeing test failures in bower where [the `instanceof` check](https://github.com/isaacs/readable-stream/blob/8625e8fc8a06866c2e389f4ee97433f465e873aa/lib/_stream_writable.js#L107) for Duplex is false when the entire test suite is run and true when a subset is run (see bower/bower#537).

I'm curious if this could be due to a difference in the way the circular dependency (`_stream_duplex` <-> `_stream_writable`) is resolved.  While I don't have a satisfactory explanation, requiring earlier (as in ff16057244d8c87dbc218a4165a0b7dec69de0fc) makes the bower tests pass consistently for me.