RepositoryRef Serializable

This is necessary to support caching of tasks that use RepsitoryRef as
inputs or outputs.

Issue gh-11158

buildSrc/src/main/java/org/springframework/gradle/github/RepositoryRef.java

@@ -1,6 +1,11 @@
 package org.springframework.gradle.github;
 
-public class RepositoryRef {
+import java.io.Serializable;
+
+public class RepositoryRef implements Serializable {
+
+	private static final long serialVersionUID = 7151218536746822797L;
+
 	private String owner;
 
 	private String name;