Don't emit regular sleeping announcements in Basic UI

They are too noisy in the Basic UI, but are still included in TTYUI.
Reported by John Wiegley.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/ui/UIBase.py

@@ -493,7 +493,7 @@ class UIBase(object):
         """
         if sleepsecs > 0:
             if remainingsecs//60 != (remainingsecs-sleepsecs)//60:
-                self.logger.info("Next refresh in %.1f minutes" % (
+                self.logger.debug("Next refresh in %.1f minutes" % (
                         remainingsecs/60.0))
             time.sleep(sleepsecs)
         return 0