Do not issue the "off" command to Matrix (#126)

This regression was introduced in PR #121 where all devices issue the "off" command to themselves upon destruction.  Using the "off" command is specifically avoided in `BuildHAT.shutdown()` with an exception built into the type identifier for the Matrix

Author: mutesplash

buildhat/devices.py

@@ -70,7 +70,8 @@ def __del__(self):
             Device._used[self.port] = False
             self._conn.callit = None
             self.deselect()
-            self.off()
+            if self._typeid != 64:
+                self.off()
 
     @property
     def _conn(self):