setup.py: pin twisted==16.1.1, remove pytrie

* To avoid an incompatible patch that landed in Twisted trunk after the
  16.1.1 release, autobahn pinned their requirement on Twisted to
  be <=16.1.1 . However Twisted reverted the patch before making a
  release. The new 16.2.0 is fine. Since autobahn has this pin, and
  since pip doesn't do full dependency resolution, I must add the pin
  too, so that 'pip install magic-wormhole' can work. I plan to remove
  this pin as soon as autobahn does the same upstream.
  https://github.com/crossbario/autobahn-python/issues/680
* A previous version of autobahn had a bug where it tried to import
  something that wasn't actually depended upon, exposed by having pynacl
  installed. Installing 'pytrie' manually fixed it. This doesn't seem to
  be a problem anymore, so I'm removing the manual dependency.

setup.py

@@ -25,10 +25,10 @@ setup(name="magic-wormhole",
                      "wormhole-server = wormhole.server.runner:entry",
                      ]},
       install_requires=["spake2==0.3", "pynacl", "argparse",
-                        "six", "twisted >= 16.1.0", "hkdf", "tqdm",
-                        "autobahn[twisted]", "pytrie",
-                        # autobahn seems to have a bug, and one plugin throws
-                        # errors unless pytrie is installed
+                        "six",
+                        "twisted==16.1.1", # since autobahn pins it
+                        "autobahn[twisted]",
+                        "hkdf", "tqdm",
                         ],
       extras_require={"tor": ["txtorcon", "ipaddr"]},
       test_suite="wormhole.test",