Store strong references to async tasks

Reason: https://news.t0.vc/IEUC

main.py

@@ -459,7 +459,7 @@ if __name__ == '__main__':
     sensors.add(ERTSCMSensor('41249312', 'Gas'))
     sensors.add(OwnTracksSensor('owntracks1', 'OwnTracks'))
     sensors.add(AirSensor('air1', 'Living Room'))
-    sensors.add(Acurite606TX('231', 'Outside'))
+    sensors.add(Acurite606TX('59', 'Outside'))
     sensors.add(AcuRite6002RM('999999', 'Seeds', 0.0))     # A
     sensors.add(AcuRite6002RM('5613', 'Misc', 0.0))     # A
     sensors.add(AcuRite6002RM('5109', 'Nook', 0.4))      # B
@@ -468,8 +468,8 @@ if __name__ == '__main__':
     sensors.add(SolarSensor('solar', 'Solar'))
 
     loop = asyncio.get_event_loop()
-    loop.create_task(poll_sensors()).add_done_callback(task_died)
-    loop.create_task(fetch_mqtt()).add_done_callback(task_died)
-    loop.create_task(run_webserver()).add_done_callback(task_died)
+    a = loop.create_task(poll_sensors()).add_done_callback(task_died)
+    b = loop.create_task(fetch_mqtt()).add_done_callback(task_died)
+    c = loop.create_task(run_webserver()).add_done_callback(task_died)
 
     loop.run_forever()