Add reference to Caffeine in cache sample

spring-boot-samples/spring-boot-sample-cache/pom.xml

@@ -91,6 +91,12 @@
 		</dependency>
 		-->
 		<!--
+		<dependency>
+			<groupId>com.github.ben-manes.caffeine</groupId>
+			<artifactId>caffeine</artifactId>
+		</dependency>
+		-->
+		<!--
 		<dependency>
 			<groupId>com.google.guava</groupId>
 			<artifactId>guava</artifactId>