Extend gym.Wrapper instead of env_modifiers

[muupan]

Since gym has introduced its own interface to modify envs using gym.Wrapper, I think it is better to use it in ChainerRL instead of directly modifying methods as in env_modifiers.

[toslunar]

It seems gym.Wrapper assumes the base environment is a subclass of gym.Env. For example,

gym.wrappers.TimeLimit(chainerrl.envs.abc.ABC(), max_episode_steps=2)

raises AttributeError: 'ABC' object has no attribute 'metadata'.