Fixed Ant never sending the done signal

[sash-a]

Noticed this when I was trying to find a fix for this issue. The bug seems to be with these two lines:

https://github.com/benelot/pybullet-gym/blob/bc68201c8101c4e30dde95f425647a0709ee2f29/pybulletgym/envs/mujoco/robots/locomotors/ant.py#L29

and

https://github.com/benelot/pybullet-gym/blob/bc68201c8101c4e30dde95f425647a0709ee2f29/pybulletgym/envs/mujoco/envs/locomotion/walker_base_env.py#L59-L60

I'm not sure if it should be changed in Ant or in WalkerBaseMuJoCoEnv not sure if other envs could benefit from this too? I made the change in Ant so as not to affect other envs.

Steps to reproduce the bug, simply run:

env = gym.make('AntMuJoCoEnv-v0')
env.render()
env.reset()
done = False
step = 0
while not done:
    _, _, done, _ = env.step([0] * 8)
    step += 1

print(step)

It should print 1000 indicating that the Ant run for the max number of steps, but its torso should have been touching the ground the whole time, which is undesired behavior.