Maildir sync crashes when .sup/sync-back-ok does not exist

Just moved to upstream/develop 43e4c1ed17e085501112c8a8b406060c5c775eb3: 

```
/home/ico/external/sup/lib/sup/util.rb:123: warning: already initialized constant EXTRACT_FIELD_NAME_RE
[2013-10-10 09:51:13 +0200] ERROR: oh crap, an exception
----------------------------------------------------------------
We are very sorry. It seems that an error occurred in Sup. Please
accept our sincere apologies. Please submit the contents of
/home/ico/.sup/exception-log.txt and a brief report of the
circumstances to https://github.com/sup-heliotrope/sup/issues so that
we might address this problem. Thank you!

Sincerely,
The Sup Developers
----------------------------------------------------------------
--- Errno::ENOENT from thread: main
No such file or directory - /home/ico/.sup/sync-back-ok
/home/ico/external/sup/lib/sup.rb:199:in `readlines'
/home/ico/external/sup/lib/sup.rb:199:in `check_syncback_settings'
bin/sup:156:in `<module:Redwood>'
bin/sup:73:in `<main>'
```

Workaround is to manually create .sup/sync-back-ok as en empty file.