Env history not managed based on history_size

[nitinsurya]

In the following locations of the code:

https://github.com/jcwleo/curiosity-driven-exploration-pytorch/blob/master/envs.py#L188-L189
https://github.com/jcwleo/curiosity-driven-exploration-pytorch/blob/master/envs.py#L286-L287

history is updated assuming the history size is 4. Shouldn't it instead be

self.history[:self.history_size-1, :, :] = self.history[1:, :, :]
self.history[self.history_size-1, :, :] = self.pre_proc(obs)

[jcwleo]

@nitinsurya
If you make PR, I'll approve that.