Remove argparse in install_requires

[davedash]

https://github.com/sivy/py-statsd/blob/master/setup.py#L17

Seems unnecessary since this is part of python since 2.7+

If you need to maintain backwards compatibility, can there be a check for python version?

It causes some annoying issues in Ubuntu:

/usr/local/lib/python2.7/dist-packages/pytz/init.py:35: UserWarning: Module argparse was already imported from /usr/lib/python2.7/argparse.pyc, but /usr/local/lib/python2.7/dist-packages is being added to sys.path
from pkg_resources import resource_stream

When using other packages that import argparse.