Make OnBeanCondition package private again

It was accidentally made public in 45b579c.

Closes gh-2741

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/OnBeanCondition.java

@@ -57,8 +57,7 @@ import org.springframework.util.StringUtils;
  * @author Stephane Nicoll
  */
 @Order(Ordered.LOWEST_PRECEDENCE)
-public class OnBeanCondition extends SpringBootCondition implements
-		ConfigurationCondition {
+class OnBeanCondition extends SpringBootCondition implements ConfigurationCondition {
 
 	private static final Log logger = LogFactory.getLog(OnBeanCondition.class);