Upgrade Spotless plugin

dependencies.gradle

@@ -8,7 +8,7 @@ ext.versions = [
 
     // Plugins
     gradlePlugin        : '3.3.2',
-    spotlessPlugin      : '3.19.0',
+    spotlessPlugin      : '3.20.0',
     versionPlugin       : '0.21.0',
     bintrayPlugin       : '0.9',