added Serializable interface to Pair class

In order to serialize an ArrayList of CommonTokens without errors

runtime/Java/src/org/antlr/v4/runtime/misc/Pair.java

@@ -30,7 +30,9 @@
 
 package org.antlr.v4.runtime.misc;
 
-public class Pair<A,B> {
+import java.io.Serializable;
+
+public class Pair<A,B> implements Serializable {
 	public final A a;
 	public final B b;