Additional condition to protect Reactor 2.0 users

(cherry picked from 80d55c4)
Closes gh-2255

Author: Dave Syer

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/reactor/ReactorAutoConfiguration.java

@@ -34,7 +34,7 @@
  * @author Dave Syer
  */
 @Configuration
-@ConditionalOnClass(EnableReactor.class)
+@ConditionalOnClass({ EnableReactor.class, Environment.class })
 @AutoConfigureAfter(WebMvcAutoConfiguration.class)
 public class ReactorAutoConfiguration {