clarified pickle version requirements (fixes #186)
diff --git a/example/example15.py b/example/example15.py
index 9868b62..1810d3e 100644
--- a/example/example15.py
+++ b/example/example15.py
@@ -14,7 +14,7 @@
 p.setExtra1(15)
 p.setExtra2(48)
 
-data = pickle.dumps(p, -1)  # -1 is important (use highest protocol version)
+data = pickle.dumps(p, 2)  # Must use pickle protocol >= 2
 print("%s %i %i" % (p.value(), p.extra1(), p.extra2()))
 
 p2 = pickle.loads(data)