hamming: change class to package private.

Fixes #1049 (#1100)

exercises/hamming/src/main/java/Hamming.java

@@ -1,4 +1,4 @@
-public class Hamming {
+class Hamming {
 
     Hamming(String leftStrand, String rightStrand) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");