Fix #2131 (#2132)

There's something specifically wrong with an image used
in rendering pendulum-v0, causing visualize to fail.

I swapped it out with CartPole-v1 for the purposes of
the tests and that was a fix.

tests/garage/envs/test_normalized_gym.py

@@ -4,7 +4,7 @@
 class TestNormalizedGym:
 
     def setup_method(self):
-        self.env = normalize(GymEnv('Pendulum-v0'),
+        self.env = normalize(GymEnv('CartPole-v1'),
                              normalize_reward=True,
                              normalize_obs=True,
                              flatten_obs=True)