Make our Clock util Serializable

It doesn't entirely make semantic sense, since the actual state of the
SystemClock isn't being preserved, but it makes injection into serializable
classes (e.g. mapreduces) much simpler, so it's worth doing.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=201755949

java/google/registry/util/SystemClock.java

@@ -25,6 +25,8 @@ import org.joda.time.DateTime;
 @ThreadSafe
 public class SystemClock implements Clock {
 
+  private static final long serialVersionUID = 5165372013848947515L;
+
   /** Returns the current time. */
   @Override
   public DateTime nowUtc() {