Bump `flatten-maven-plugin` from 1.3.0 to 1.4.0

This should level the differences between Windows and UNIX.
diff --git a/pom.xml b/pom.xml
index ebd9971..1fab382 100644
--- a/pom.xml
+++ b/pom.xml
@@ -67,7 +67,7 @@
     <project.build.outputTimestamp>0</project.build.outputTimestamp>
 
     <!-- plugin versions -->
-    <flatten-maven-plugin.version>1.3.0</flatten-maven-plugin.version>
+    <flatten-maven-plugin.version>1.4.0</flatten-maven-plugin.version>
     <maven-deploy-plugin.version>3.0.0</maven-deploy-plugin.version>
     <sign-maven-plugin.version>1.0.1</sign-maven-plugin.version>