Depend upon autobahn, for upcoming websocket support.

Use 'autobahn[twisted]' just to be sure (plain 'autobahn' worked fine
for py27, but maybe it's needed for py35 or something).

Autobahn is failing to do some conditional import and accidentally
depends upon pytrie (for some encrypted WAMP thing) when we didn't ask
for it (https://github.com/crossbario/autobahn-python/issues/604). This
commit also adds a manual dependency on pytrie (which is pretty small)
until the upstream bug is fixed.

setup.py

@@ -24,7 +24,11 @@ setup(name="magic-wormhole",
                      "wormhole-server = wormhole_server.runner:entry",
                      ]},
       install_requires=["spake2==0.3", "pynacl", "requests", "argparse",
-                        "six", "twisted >= 16.1.0", "hkdf"],
+                        "six", "twisted >= 16.1.0", "hkdf",
+                        "autobahn[twisted]", "pytrie",
+                        # autobahn seems to have a bug, and one plugin throws
+                        # errors unless pytrie is installed
+                        ],
       extras_require={"tor": ["txtorcon", "ipaddr"]},
       test_suite="wormhole.test",
       cmdclass=commands,