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.

```
$ pundler install
processing requirements.in
Caught AttributeError while processing 'setuptools'
Caught AttributeError while processing 'PasteDeploy'
Caught AttributeError while processing 'setuptools'
dependency WebOb==1.4
dependency pyramid==1.4.2
dependency translationstring==1.1
dependency repoze.lru==0.6
dependency zope.deprecation==4.1.1
dependency Chameleon==2.16
dependency MarkupSafe==0.23
dependency Mako==0.9.1
dependency venusian==1.0a8
dependency zope.interface==4.1.1
dependency jinja2==2.7.2
dependency markupsafe==0.23
dependency jinja2==2.7.2
dependency twisted==14.0.0
dependency zope.interface==4.1.1
dependency markupsafe==0.23
dependency txtemplate==1.0.3
```