update version to 1.6.0 to avoid confusion

dtool/PandaVersion.pp

@@ -3,7 +3,7 @@
 // ppremake scripts for Panda.
 
 // Use spaces to separate the major, minor, and sequence numbers here.
-#define PANDA_VERSION 1 5 4
+#define PANDA_VERSION 1 6 0
 
 // This variable will be defined to false in the CVS repository, but
 // scripts that generate source tarballs and/or binary releases for