Update BaseEntity.java

This fix the following error :

   "setAttribute: Non-serializable attribute owner"

In the following context :

  - hsql database configuration (updating data-access.properties only)

  - mvn clean install bees:run (cloudbees local run)

src/main/java/org/springframework/samples/petclinic/model/BaseEntity.java

@@ -27,7 +27,7 @@ import javax.persistence.MappedSuperclass;
  * @author Juergen Hoeller
  */
 @MappedSuperclass
-public class BaseEntity {
+public class BaseEntity implements Serializable {
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     protected Integer id;