Upgrade to Reactor Bismuth SR15

Author: jhoeller

Diff for: build.gradle

@@ -48,7 +48,7 @@ ext {
 	kotlinVersion        = "1.2.71"
 	log4jVersion         = "2.11.1"
 	nettyVersion         = "4.1.32.Final"
-	reactorVersion       = "Bismuth-SR14"
+	reactorVersion       = "Bismuth-SR15"
 	rxjavaVersion        = "1.3.8"
 	rxjavaAdapterVersion = "1.2.1"
 	rxjava2Version       = "2.1.17"