update requirements.txt and setup.py for gym

[keisuke-nakata]

The latest version of gym dropped support of underscore methods in env_wrappers like _observation.
ChainerRL now implements underscore methods, we need to fix the version of gym.

Here is my installation log (at a brand new virtualenv):

$ python setup.py install
*snip*

$ pip freeze
cached-property==1.5.1
certifi==2019.3.9
chainer==7.0.0a1
chainerrl==0.6.0
chardet==3.0.4
filelock==3.0.12
future==0.17.1
gym==0.12.1
idna==2.8
numpy==1.16.4
Pillow==6.0.0
protobuf==3.7.1
pyglet==1.4.0b1
requests==2.22.0
scipy==1.3.0
six==1.12.0
typing==3.6.6
typing-extensions==3.7.2
urllib3==1.25.3