render_modes compatability with gym>0.17.3 (#2334)

Author: ahalev

src/garage/envs/gym_env.py

@@ -144,7 +144,10 @@ def __init__(self, env, is_image=False, max_episode_length=None):
         self._max_episode_length = _get_time_limit(self._env,
                                                    max_episode_length)
 
-        self._render_modes = self._env.metadata['render.modes']
+        try:
+            self._render_modes = self._env.metadata['render_modes']
+        except KeyError:
+            self._render_modes = self._env.metadata['render.modes']
 
         self._step_cnt = None
         self._visualize = False