Fixed issue with unity environment not being found on MacOS (#236)

If the internal executable can't be found, it will look for any file in the folder and run it.

python/unityagents/environment.py

             candidates = glob.glob(os.path.join(cwd, file_name + '.app', 'Contents', 'MacOS', true_filename))
             if len(candidates) == 0:
                 candidates = glob.glob(os.path.join(file_name + '.app', 'Contents', 'MacOS', true_filename))
+            if len(candidates) == 0:
+                candidates = glob.glob(os.path.join(cwd, file_name + '.app', 'Contents', 'MacOS', '*'))
+            if len(candidates) == 0:
+                candidates = glob.glob(os.path.join(file_name + '.app', 'Contents', 'MacOS', '*'))
             if len(candidates) > 0:
                 launch_string = candidates[0]
         elif platform == 'win32':