Update to Groovy 2.4.20

Closes gh-8931

gradle/dependency-management.gradle

@@ -135,9 +135,9 @@ dependencyManagement {
 		dependency 'org.attoparser:attoparser:2.0.4.RELEASE'
 		dependency 'org.bouncycastle:bcpkix-jdk15on:1.64'
 		dependency 'org.bouncycastle:bcprov-jdk15on:1.64'
-		dependency 'org.codehaus.groovy:groovy-all:2.4.19'
-		dependency 'org.codehaus.groovy:groovy-json:2.4.19'
-		dependency 'org.codehaus.groovy:groovy:2.4.19'
+		dependency 'org.codehaus.groovy:groovy-all:2.4.20'
+		dependency 'org.codehaus.groovy:groovy-json:2.4.20'
+		dependency 'org.codehaus.groovy:groovy:2.4.20'
 		dependency 'org.eclipse.jdt:ecj:3.12.3'
 		dependency 'org.eclipse.jetty.websocket:websocket-api:9.4.7.v20170914'
 		dependency 'org.eclipse.jetty.websocket:websocket-client:9.4.7.v20170914'