Only require argparse package for python versions < 2.7


Python 2.7 has argparse built in, so having it as a dependency causes this error:

```
/usr/local/share/python/shovel:26: UserWarning: Module argparse was already imported from /usr/local/Cellar/python/2.7.4/Frameworks/Python.framework/Versions/2.7/lib/python2.7/argparse.pyc, but /usr/local/lib/python2.7/site-packages is being added to sys.path
```