fix issue #100 with new Box2D-kengz dependency

[kengz]

This fixes #100 Box2d won't find some RAND_LIMIT_swigconstant

The issue has proven a hurdle for users who wants to get started on the gym, or for developers who builds projects with gym as the dependency since a clean install will have the broken 2d environments.

Steps:

• clone the Box2D repo, release a fixed version to PyPi as Box2D-kengz at version 2.3.3 (since the author seems inactive). The fork is here.
• update gym's dependency in setup.py to point to this package
• test the installation with LunarLander-v2 environment on direct install of gym from my fork. Issue resolved.

[andrewschreiber]

It's a good move to switch to Kengz's fork as the original PyBox2D maintainer @kne has not merged a PR for several months and is seemingly unreachable. A PR to fix the issue on PyBox2D has been open since February 14th.

I can say that Kengz's pip install -e git+https://github.com/kengz/gym.git#egg=gym[all] made my LunarLander-v2 environment work perfectly.

[danijar]

@gdb Is there anything that keeps this from getting merged?

[gdb]

This is great. Thank you @kengz!