Log Message:
mail/offlineimap: Add dependency on net/py-rfc6555

From J. Lewis Muir via wip.

OfflineIMAP requires the rfc6555 module, but the package is missing the
dependency declaration.  Without rfc6555, OfflineIMAP builds but does
not run; it fails with a stack trace that ends like this:

  File "[...]/offlineimap/imaplibutil.py", line 22, in <module>
      import rfc6555
      ImportError: No module named rfc6555