Bump version number to 3.9.1-SNAPSHOT

build.gradle

@@ -7,7 +7,7 @@ plugins {
 
 allprojects {
   project.group = 'de.hhu.stups'
-  project.version = "3.9.0"
+  project.version = "3.9.1-SNAPSHOT"
   project.ext.isSnapshot = project.version.endsWith("-SNAPSHOT")
 
   ext."signing.secretKeyRingFile" = rootProject.file("secring.gpg").absolutePath