Bump version to v3.1.0-rc1

Change-Id: I7dca20ef063f6347b04c071575bb01f87955f9c1
diff --git a/build.gradle b/build.gradle
index f905e80..5574fe3 100644
--- a/build.gradle
+++ b/build.gradle
@@ -5,4 +5,4 @@
 }
 
 // Zentrale Projektversion für korapxmltool
-version = '3.0.0'
+version = '3.1.0-rc1'