BUG: Stops the TradingEnvironment from being created at import time when

zipline.utils.events is imported.

Changes the class level attribute `env` on EventRule to a property so
that the environment is only looked up at when needed.

zipline/utils/events.py

@@ -175,7 +175,9 @@ class EventRule(six.with_metaclass(ABCMeta)):
     """
     An event rule checks a datetime and sees if it should trigger.
     """
-    env = TradingEnvironment.instance()
+    @property
+    def env(self):
+        return TradingEnvironment.instance()
 
     @abstractmethod
     def should_trigger(self, dt):