Undefined names 'mujoco' and 'SubprocVecEnv'

[cclauss]

flake8 testing of https://github.com/uber-common/deep-neuroevolution on Python 3.6.3

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./gpu_implementation/gym_tensorflow/__init__.py:13:16: F821 undefined name 'mujoco'
        return mujoco.HumanoidMO('Humanoid-v1', batch_size, *args, **kwargs)
               ^
./gpu_implementation/gym_tensorflow/__init__.py:15:16: F821 undefined name 'SubprocVecEnv'
        return SubprocVecEnv([lambda: make(game[4:], 1, *args, **kwargs) for _ in range(batch_size)])
               ^
2     F821 undefined name 'mujoco'
2

[fps7806]

Those classes are still under-development and shouldn't be referenced in the master branch. I removed the references.