Strictly require Twisted 12.2.0.

This is a temporary fix for issue ooni#28
The problem arises from the fact that we are overriding a private method
(_WrappingProtocol that has been removed in 12.3.0). The proper way to fix this
would be to get rid of the hack and use a HTTP Connection Pool.

requirements.txt

@@ -1,6 +1,6 @@
 PyYAML>=3.10
 Pygments>=1.5
-Twisted>=12.1.0
+Twisted==12.2.0
 argparse>=1.2.1
 distribute>=0.6.24
 dnspython>=1.10.0