Fix Machine ui to not error out on ui.warn()

All other uis (especially BaseUI) define as warn(self, msg, minor = 0)
just MachineUI required minor without a default. This leads the
Machine UI to error out with an exception if we pass it
ui.warn("string") which is the common thing in our code base. This
patch is therefore small but critical in fixing this UI.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/ui/Machine.py

@@ -56,7 +56,7 @@ class MachineUI(UIBase):
     def _display(s, msg):
         s._printData('_display', msg)
 
-    def warn(s, msg, minor):
+    def warn(s, msg, minor = 0):
         s._printData('warn', '%s\n%d' % (msg, int(minor)))
 
     def registerthread(s, account):