Migrate to mujoco-py 1.50 (openai#834)

* all envs run offscreen

* render works

* changed mujoco-py version

* Bump versions

* Update version and README

* Same versioning for all mujoco envs

* Fix typo

* Fix version

* Bump version again

* Revert "Fix version"

This reverts commit decc577.

Cut release for v0.7.4

Bump version