Change install_requires version restrictions

[paazmaya]

Instead of locking to a certain version, allow the user to have different patch version of the required libraries.

For example, instead of requiring only 2.2.1

'Pillow==2.2.1'

Allow the user to have anything that starts with 2.2, like the latest 2.2.2 of the given Pillow (PIL fork), which itself is a bit tricky to get installed while in Windows x64.

'Pillow>=2.2'

http://www.python.org/dev/peps/pep-0345/#version-specifiers