Use version from setuptools-git-version instead of hard coding

[asbjornu]

setuptools-git-version allows us to use the version number exposed by Git instead of hard coding it to disk. Since we need this version number only when we deploy a new version of the pypi package and Travis only will deploy tagged commits, we can basically just use the Git tag as the version number. This makes releasing even simpler, since we don't have to repeat the version number in both Git tag and inside the payex init file.