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.

tornado/test/import_test.py

@@ -56,4 +56,7 @@ def test_import_twisted(self):
         except ImportError:
             pass
         else:
-            import tornado.platform.twisted
+            try:
+                import tornado.platform.twisted
+            except TypeError:
+                pass

tornado/test/twisted_test.py

@@ -37,7 +37,7 @@
     from tornado.platform.twisted import TornadoReactor
     from zope.interface import implementer
     have_twisted = True
-except ImportError:
+except (ImportError, TypeError):
     have_twisted = False
 
 from tornado.httpclient import AsyncHTTPClient