Change the requirements to use the snapshot build of the
requests library that we need.
When trying to install requests-kerberos setup.py cannot import
requets_kerberos.  Doing this causes a problem where per
requirements.txt, kerberos is downloaded but not yet installed when
requests-kerberos's setup.py is evaluated.  Since setup.py imports
requests_kerberos, and requests_kerberos import kerberos, the
installation fails.

See https://gist.github.com/pcn/5209734 for an example.

This commit fixes the problem by using the compiler module to parse
__init__.py, but doesn't evaluate it.  It then searches for the node
whose name is __version__ and uses the constant that would be assigned
to it for setup.py.