Make SQSEvent.SQSMessage default ctor public

Author: jeremywall

aws-lambda-java-events/src/main/java/com/amazonaws/services/lambda/runtime/events/SQSEvent.java

@@ -227,7 +227,7 @@ public static class SQSMessage implements Serializable, Cloneable {
         /**
          * Default constructor
          */
-        private SQSMessage() {}
+        public SQSMessage() {}
 
         /**
          * Gets the message id