Merge pull request #1057 from bmkiefer/accumulate-ref-solution

accumulate: make classes and methods package private in reference solution

exercises/accumulate/.meta/src/reference/java/Accumulate.java

 import java.util.List;
 import java.util.function.Function;
 
-public class Accumulate {
+class Accumulate {
 
-	public static <T> List<T> accumulate(List<T> collection, Function<T, T> function) {
+	static <T> List<T> accumulate(List<T> collection, Function<T, T> function) {
 		List<T> newCollection = new ArrayList<>();
 
 		for (T item : collection) {