from gym.wrappers import FlattenDictWrapper of cmd_util.py in ~/baselines/baselines/common is not correct

[HeegerGao]

The 12th line in cmd_util.py file is not correct:
from gym.wrappers import FlattenDictWrapper
This is out of date after gym 0.15.4. I think the correct code is:
try:
from gym.wrappers import FlattenDictWrapper
except:
# gym 0.15.4 remove FlattendDictWrapper
from gym.wrappers import FilterObservation, FlattenObservation