update ars.py and README to reflect changes in ray 1.0.

[sgillen]

While trying to run some examples to benchmark this code, I had to make a few small changes to work with ray 1.0, and to reflect that the mujoco 1.0 envs are no longer available in current versions of mujoco_py.

Not sure this really needs to be merged, but will be useful to anyone who wants to run this code in 2020.