Disable tests when old versions of twisted are installed.
Twisted 10.0 (on an older version of Ubuntu) doesn't seem to work with class decorators, even on python 2.6. This makes the tests fail with a TypeError: Traceback (most recent call last): File "tornado/test/import_test.py", line 59, in test_import_twisted import tornado.platform.twisted File "tornado/platform/twisted.py", line 108, in <module> TornadoDelayedCall = implementer(IDelayedCall)(TornadoDelayedCall) File "/usr/lib/python2.6/dist-packages/zope/interface/declarations.py", line 496, in __call__ raise TypeError("Can't use implementer with classes. Use one of " TypeError: Can't use implementer with classes. Use one of the class-declaration functions instead. If we catch a typeerror while importing twisted, act like twisted is not installed.