Prepare for 2.81 release, increase version numbers

Fixed DNA serialization for C++11: avoid negative char values
Fixes 617. (needs to be tested)

src/LinearMath/btSerializer.h

@@ -438,7 +438,7 @@ public:
 
 			buffer[9] = '2';
 			buffer[10] = '8';
-			buffer[11] = '0';
+			buffer[11] = '1';
 
 		}