Fixed issue with checking of Cython version

Fixing the problem with checking for the Cython version. The Cython package in Ubuntu 14.04 is in version 0.20.1post0 that cause following issue:
@jakubkrajniak ForcePy $ python setup.py install --user
Downloading http://pypi.python.org/packages/source/d/distribute/distribute-0.6.14.tar.gz
Extracting in /tmp/tmphe0Pnp
Now working in /tmp/tmphe0Pnp/distribute-0.6.14
Building a Distribute egg in /home/jakub/project/software/ForcePy
/home/jakub/project/software/ForcePy/distribute-0.6.14-py2.7.egg
Traceback (most recent call last):
  File "setup.py", line 59, in <module>
    if not int(Cython.__version__.replace(".","")) >= int(required_version.replace(".", "")):
ValueError: invalid literal for int() with base 10: '0201post0'