Add spring-messaging to the websocket starter

Fixes gh-1241

Author: philwebb

spring-boot-starters/spring-boot-starter-websocket/pom.xml

@@ -36,6 +36,10 @@
 				</exclusion>
 			</exclusions>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework</groupId>
+			<artifactId>spring-messaging</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework</groupId>
 			<artifactId>spring-websocket</artifactId>