Add non-interactive pendulum environment.

[aterenin]

Often times, it's useful to have environments that have no actions for debugging purposes. This adds a simple pendulum as such an environment.

[aterenin]

This will fail CI because it relies on upstream changes in RLBase here: https://github.com/JuliaReinforcementLearning/ReinforcementLearningBase.jl/pull/26/files

[findmyway]

I guess that for all the NonInteractiveEnvs, the only acceptable action is nothing?

If that is the case, can we also add another method like env() = env(nothing)?

[aterenin]

Yeah, this is the idea. Basically you set a policy that always returns nothing. I'll add this syntax.

I got the tests to run, and it turns out this PR fails because due to a type error I just fixed upstream. Let me get this into a passing state.

[aterenin]

Okay, looks like this fails CI due to upstream changes not yet being visible on Travis. On my end, I'm seeing the following (other tests are commented).

(ReinforcementLearningEnvironments) pkg> test
   Testing ReinforcementLearningEnvironments
 Resolving package versions...
[ Info: Testing MountainCarEnv()
[ Info: Testing ContinuousMountainCarEnv()
[ Info: Testing PendulumEnv()
[ Info: Testing PendulumNonInteractiveEnv()
Test Summary:                     | Pass  Total
ReinforcementLearningEnvironments | 1212   1212
   Testing ReinforcementLearningEnvironments tests passed 

[findmyway]

Thanks. Merged