bump version to 1.13.1

diff --git a/Makefile b/Makefile
index b50c722..a7934c8 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 VERSION = 1
 PATCHLEVEL = 13
-SUBLEVEL = 0
+SUBLEVEL = 1
 EXTRAVERSION =
 NAME = Unnamed