Running Differential Programming Models

I've been trying to run the differential programming models and have been running into quite a few issues.

Ex
cartpole/DiffRL.jl

- Seems to require julia 1.1, but not documented that I can find
- Gym specified in manifest/project doesn't seem correct.
- Is this model described anywhere in more detail?

cartpole/DQN.jl

- In addition to DiffRL's problems, the following
- A few additional deps not described in manifest/Project
- ERROR: LoadError: Can't differentiate `setindex!`

If you'd like a dockerfile for it I'll submit a pr, I'm not sure how to get past setindex! problem though and imagine it's a specific version I haven't specified.

This is some awesome work, definetly planning to continue exploring differential programming more. Seems extremely valuable.

Start by running the models in cartpole/DiffRL.jl and cartpole/DQN.jl and compare their Julia and dependency requirements with the repository manifests. Investigate the reported setindex! differentiation error and the missing dependencies. Done means the models have reproducible setup instructions and the reported execution problems are resolved or clearly documented.

Written by the indexing model from the issue text.