Replace rllab.envs.Env with gym.Env

[ryanjulian]

The community has settled on gym.Env as a de-facto standard environment interface. There's no reason to keep our own around.

The scope of this change is to remove the rllab.envs.Env base interface, and refactor implementing classes to instead implement gym.Env. Note that this explicitly does not mean that we are adopting the physics engine, registration system, benchmarks, etc of OpenAI Gym--just the gym.Env abstract interface.

[ryanjulian]

See rlworkgroup/garage#12