Remove upper bound for Pillow in setup.py

Hi,

I think it is better to remove upper bound for Pillow in setup.py. Being defensive is good in requirements.txt, but having `Pillow < 2.4` in `install_requires` may disallow some valid setups (mitmproxy 0.10 and Pillow > 2.4) even if they will work fine. I think it may also cause Pillow to be downgraded for some users.