update Atari envs to v4 and warn Python 2 users.

setup.py

@@ -1,14 +1,15 @@
 from setuptools import setup, find_packages
-import os
-
-
-repo_dir = os.path.dirname(os.path.abspath(__file__))
+import sys
 
+if sys.version_info.major != 3:
+    print("This Python is only compatible with Python 3, but you are running "
+          "Python {}. The installation will likely fail.".format(sys.version_info.major))
 
 setup(name='baselines',
       packages=[package for package in find_packages()
                 if package.startswith('baselines')],
       install_requires=[
+          'gym',
           'scipy',
           'tqdm',
           'joblib',
@@ -22,4 +23,4 @@ setup(name='baselines',
       author="OpenAI",
       url='https://github.com/openai/baselines',
       author_email="gym@openai.com",
-      version="0.1.0")
+      version="0.1.3")