Update ez_setup.py

Copied verbatim from https://bootstrap.pypa.io/ez_setup.py
Always build Cython-generated sources if Cython is new enough

Print a more detailed error message if the Cython-generated sources are
absent and the user does not have a new enough version of Cython to
generate them.
Merge pull request #178 from lpsinger/use_setuptools_32

Always build Cython-generated sources if Cython is new enough