Update minimum version jplephem>=2.13 needs in setup for skyfield

[mworion]

Hi Brandon,
another topic reached me on ubuntu:

your setup.py for skyfield says actually jplephem>=2.11
but in actual skyfield you import in jpllib line 6:

from jplephem.exceptions import OutOfRangeError

which is implemented only from jplephem from version 2.13 on. This leads to an "Module not found error".

Michel

[brandon-rhodes]

Thanks for letting me know! I've updated the requirement, and plan to make a release today or tomorrow, which will publish the more accurate requirement.