Bump version to v3.1.3

Change-Id: Iace5f24d2a24f2eee45279ad1b33b59b6fa994e1
diff --git a/build.gradle b/build.gradle
index 2b3bb08..ddef8b4 100644
--- a/build.gradle
+++ b/build.gradle
@@ -5,4 +5,4 @@
 }
 
 // Zentrale Projektversion für korapxmltool
-version = '3.1.2'
+version = '3.1.3'