Upgrade gym to 0.22.0

[RohanM]

This PR is based on the following, which should be reviewed and merged first:

• Explicitly specify gym version 0.20.0 #15
• Upgrade gym to 0.21.0 #16

Continue the incremental upgrade path by upgrading gym to 0.22.0. In this release, gym.make() no longer accepts the gym's package name. So far we've done this:

env = gym.make('gym_go:go-v0')

But now we need to:

import gym_go
env = gym.make('go-v0')