Upgrade to RxJava 2.2.4

Author: jhoeller

build.gradle

@@ -41,7 +41,7 @@ ext {
 	reactorVersion       = "Californium-SR3"
 	rxjavaVersion        = "1.3.8"
 	rxjavaAdapterVersion = "1.2.1"
-	rxjava2Version       = "2.2.3"
+	rxjava2Version       = "2.2.4"
 	slf4jVersion         = "1.7.25"	  // spring-jcl + consistent 3rd party deps
 	tiles3Version        = "3.0.8"
 	tomcatVersion        = "9.0.13"