Bump version number to 3.1.1

Signed-off-by: Andreas Fuchs <andreas.fuchs@sit.fraunhofer.de>
diff --git a/configure.ac b/configure.ac
index 7963d17..32e9658 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4,7 +4,7 @@
 # All rights reserved.
 
 AC_INIT([tpm2-tss],
-        [3.1.0],
+        [3.1.1],
         [https://github.com/tpm2-software/tpm2-tss/issues],
         [],
         [https://github.com/tpm2-software/tpm2-tss])