Ensure CompositeDevice._order has deterministic behaviour

... by sorting the kwargs, if no _order was specifically supplied.

gpiozero/devices.py

@@ -261,7 +261,7 @@ class CompositeDevice(Device):
         self._tuple = None
         self._order = kwargs.pop('_order', None)
         if self._order is None:
-            self._order = kwargs.keys()
+            self._order = sorted(kwargs.keys())
         self._order = tuple(self._order)
         for missing_name in set(kwargs.keys()) - set(self._order):
             raise CompositeDeviceBadOrder('%s missing from _order' % missing_name)