Update dependencies to their latest versions.

This PR gets me a clean build and test with the latest versions of our various dependencies, modulo two upstream bugs in PyZMQ:

  https://github.com/zeromq/pyzmq/pull/462
  https://github.com/zeromq/pyzmq/pull/463

Mostly just bumping version numbers, but some key points include:

  * new gevent version should install cleanly without a helper script
  * gevent_zeromq is now deprecated, so we have to do our own monkey-patching on top of zmq.green
  * new WebOb throws an error if header values are unicode strings, so we have to encode some BrowserID assertions into bytestrings
  * pypi mirrors are gone so the Makefile is tweaked accordingly

If we don't get a quick response on the upstream bugs I will look at installing from a fork in the meantime.

@ametaireau r?