Pin cryptography in the twisted tox tests

The latest version breaks the twisted tests.

Author: pgjones

Diff for: tox.ini

@@ -15,6 +15,7 @@ commands= py.test {posargs} {toxinidir}/test/
 # This is a validation test that confirms that Twisted's test cases haven't
 # broken.
 deps =
+    cryptography==2.4.2
     twisted[tls, http2, conch]
     sphinx
 commands = python -m twisted.trial --reporter=text twisted