[modules/battery-upower] Fix capacity default value

If the capacity isn't set, get() returns None, which is not comparable.

Use -1 instead, which makes the capacity unknown.

fixes #555

bumblebee/modules/battery-upower.py

@@ -240,7 +240,7 @@ class Module(bumblebee.engine.Module):
 
     def state(self, widget):
         state = []
-        capacity = widget.get("capacity")
+        capacity = widget.get("capacity", -1)
         if capacity < 0:
             return ["critical", "unknown"]