Wrapped import pybullet_envs statement in utils.py (#40)

* wrapped if statement in utils.py to allow enjoy.py to run without pybullet dependency on atari environments

* restoring benchmark.md after test suite

* respecting formatting

utils/utils.py

@@ -6,7 +6,11 @@
 import importlib
 
 import gym
-import pybullet_envs
+try:
+    import pybullet_envs
+except ImportError:
+    pybullet_envs = None
+
 from gym.envs.registration import load
 
 from stable_baselines.deepq.policies import FeedForwardPolicy