Fixing MiniHacks obs space if include_alignment_blstats==False

[samvelyan]

observation_space was being computed incorrectly when include_alignment_blstats=False was being passed to MiniHack environments, introduced in #77.

This PR makes sure that the observation_space is computed accurately.

>>> import gym, minihack
>>> env = gym.make("MiniHack-MultiRoom-N6-v0", include_alignment_blstats=False)
>>> env.observation_space["blstats"]
Box(-2147483648, 2147483647, (26,), int64)
>>> env = gym.make("MiniHack-MultiRoom-N6-v0", include_alignment_blstats=True)
>>> env.observation_space["blstats"]
Box(-2147483648, 2147483647, (27,), int64)