fix import issue with newer versions of pybullet

[isacarnekvist]

Remaining issue that was partly fixed:
Issue: #36
Partal fix: #35

I just made the same one-liner fix for the mujoco envs. Tried it with pybullet 2.7.9 and it works.

Tests

Very little manual testing done, just opened an environment that previously did not work --- now it works.

Also ran:

python pybulletgym/tests/test_pybulletgym_mujoco_sanity.py

which reported no problems.

On the other hand:

python pybulletgym/tests/test_pybulletgym_roboschool_performance.py

showed this (before fix also):

The following envs perform worse: ['Walker2DPyBulletEnv-v0', 'HumanoidFlagrunHarderPyBulletEnv-v0', 'AtlasPyBulletEnv-v0'] 

The following envs have problems: ['PusherPyBulletEnv-v0', 'ThrowerPyBulletEnv-v0', 'StrikerPyBulletEnv-v0']

[benelot]

The tests show that everything runs as expected. I tried to build a performance related test with roboschool performance, but that should not change with your import. No idea why it did not work as expected.

[benelot]

I will merge as it its the correct fix to apply.