Add "DNE" to GymExperiment

[daveliepmann]

Apologies for not opening an issue first, but this seemed to be a small fix. Ruby is not my area of expertise so hopefully I don't misunderstand the problem.

I get the following error running the Acrobot experiment (with bundle exec ruby experiments/acrobot.rb) as-is:

experiments/acrobot.rb:23:in `<main>': uninitialized constant GymExperiment (NameError)

Changing that line to add a DNE:: prefix to GymExperiment removes this error.