Fix NoClassDefFoundError loading RxJavaAutoConfiguration without spring-webmvc (#1442)

RxJavaReturnValueHandlerConfig can only be loaded when both spring-web
and spring-webmvc are on the classpath, because it contains a reference
to the WebMvcConfigurerAdapter class.

But the ConditionalOnClass annotation only included the class from
spring-web. In our situation, only spring-web was on the classpath,
resulting in a NoClassDefFoundError for WebMvcConfigurerAdapter. Fix it
by adding that class to the condition as well.

spring-cloud-netflix-core/src/main/java/org/springframework/cloud/netflix/rx/RxJavaAutoConfiguration.java

@@ -39,7 +39,7 @@ import rx.Single;
 public class RxJavaAutoConfiguration {
 
 	@Configuration
-	@ConditionalOnClass(AsyncHandlerMethodReturnValueHandler.class)
+	@ConditionalOnClass({ AsyncHandlerMethodReturnValueHandler.class, WebMvcConfigurerAdapter.class })
 	protected static class RxJavaReturnValueHandlerConfig {
 		@Bean
 		public SingleReturnValueHandler singleReturnValueHandler() {