Fix AttributeError for None/endswith when installing setuptools

pundler is choking for me on setuptools for some reason

```python
$ pundler install
processing requirements.in
Traceback (most recent call last):
  File "/Users/marca/python/virtualenvs/pundler/bin/pundler", line 10, in <module>
    execfile(__file__)
  File "/Users/marca/dev/git-repos/pundler/bin/pundler", line 3, in <module>
    main()
  File "/Users/marca/dev/git-repos/pundler/pundler/core.py", line 120, in main
    args.func(args)
  File "/Users/marca/dev/git-repos/pundler/pundler/core.py", line 79, in install
    deps[line].append("%s==%s" % (package.name, package.installed_version))
  File "/Users/marca/python/virtualenvs/pundler/lib/python2.7/site-packages/pip/req.py", line 455, in installed_version
    return self.pkg_info()['version']
  File "/Users/marca/python/virtualenvs/pundler/lib/python2.7/site-packages/pip/req.py", line 424, in pkg_info
    logger.warn('No PKG-INFO file found in %s' % display_path(self.egg_info_path('PKG-INFO')))
  File "/Users/marca/python/virtualenvs/pundler/lib/python2.7/site-packages/pip/req.py", line 371, in egg_info_path
    base = os.path.join(self.source_dir, 'pip-egg-info')
  File "/Users/marca/python/virtualenvs/pundler/bin/../lib/python2.7/posixpath.py", line 77, in join
    elif path == '' or path.endswith('/'):
AttributeError: 'NoneType' object has no attribute 'endswith'
```

This PR catches the `AttributeError` and allows it to continue.