Fix wrong urls built by using bundle.urls instead of bundle.build

These are related errors:
```
  File "/Users/keitheis/.virtualenvs/34/lib/python3.4/site-packages/webassets/cache.py", line 82, in walk
    raise ValueError('Cannot MD5 type %s' % type(obj))
ValueError: Cannot MD5 type <class 'method'>
```
or in browser console the url looks like
```
http://0.0.0.0:6543/%3CFileHunk%20/Users/dog/app/static/build/css/app.build/css/app.%(version)s.css.css%3E
```