Fix inverted button logic. (#288)

This is needed to work with the latest kernel.
See #286

Author: ensonic

ev3dev/core.py

@@ -2555,7 +2555,7 @@ def buttons_pressed(self):
         for k, v in self._buttons.items():
             buf = self._buffer_cache[v['name']]
             bit = v['value']
-            if not bool(buf[int(bit / 8)] & 1 << bit % 8):
+            if bool(buf[int(bit / 8)] & 1 << bit % 8):
                 pressed += [k]
         return pressed