Fix Eureka Ribbon Auto Config conditions.

Specifically, add `@ConditionalOnBean(EurekaClient.class)`.

Fixes an issue where eureka and ribbon are on the classpath, eureka.client.enabled=false which would cause a bean not found exception.

fixes gh-1511

(cherry picked from commit d1f8c03)

spring-cloud-netflix-eureka-server/pom.xml

@@ -37,6 +37,11 @@
 			<groupId>org.springframework.cloud</groupId>
 			<artifactId>spring-cloud-commons</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.cloud</groupId>
+			<artifactId>spring-cloud-context</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.cloud</groupId>
 			<artifactId>spring-cloud-netflix-core</artifactId>