Upgrade to Netty 4.1.20

Author: jhoeller

build.gradle

@@ -52,7 +52,7 @@ configure(allprojects) { project ->
 	ext.junitVintageVersion  = "4.12.3"
 	ext.kotlinVersion        = "1.2.20"
 	ext.log4jVersion         = "2.10.0"
-	ext.nettyVersion         = "4.1.19.Final"
+	ext.nettyVersion         = "4.1.20.Final"
 	ext.reactorVersion       = "Bismuth-BUILD-SNAPSHOT"
 	ext.rxjavaVersion        = "1.3.4"
 	ext.rxjavaAdapterVersion = "1.2.1"