Restore Travis workaround. [skip ci]

greentest/test__server.py

@@ -189,6 +189,10 @@ class TestCase(greentest.TestCase):
             self.assert_error(TypeError)
         finally:
             self.server.stop()
+            # XXX: There's something unreachable (with a traceback?)
+            # We need to clear it to make the leak checks work on Travis;
+            # so far I can't reproduce it locally on OS X.
+            import gc; gc.collect()
 
     def ServerClass(self, *args, **kwargs):
         kwargs.setdefault('spawn', self.get_spawn())