Fix requires for python2-six
    
    Otherwise we get the following traceback:
    
      Traceback (most recent call last):
        File "/usr/bin/offlineimap", line 20, in <module>
          from offlineimap import OfflineImap
        File "/usr/lib/python2.7/site-packages/offlineimap/__init__.py", line 20, in <module>
          from offlineimap.init import OfflineImap
        File "/usr/lib/python2.7/site-packages/offlineimap/init.py", line 34, in <module>
          from offlineimap import threadutil, accounts, folder, mbnames
        File "/usr/lib/python2.7/site-packages/offlineimap/accounts.py", line 23, in <module>
          import six
      ImportError: No module named six
    
    Signed-off-by: Paul Belanger <pabelanger@redhat.com>