Explicitly specify gym version 0.20.0

[RohanM]

OpenAI Gym has a number of breaking changes. This gym isn't yet compatible with them, so let's specify the latest version that works.

Steps to reproduce

1. git checkout master
2. Install packages in an isolated venv:

rm -r screenshots # interferes with packaging process
pipenv install -e .
pipenv install pyglet scipy sklearn
pipenv shell
python demo.py

3. You should see an error initialising the gym: gym.error.NameNotFound: Environment 'gym_go:go' doesn't exist.

4. Reset the repository and check out this branch:

pipenv --rm
rm -r Pipfile* build

5. Repeat this process
6. The demo should start