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.