Upgrade of maven plugins (#951)

* Upgrade maven plugins

* Upgrade maven plugins

Some general code cleanup was necessary due to upgrade of PMD and checkstyle.

Also needed to add Junit 4 as a dependency due to Mockito.timout issue found here:
https://github.com/mockito/mockito/issues/152

double-checked-locking/src/main/java/com/iluwatar/doublechecked/locking/App.java

@@ -55,7 +55,9 @@ public class App {
     ExecutorService executorService = Executors.newFixedThreadPool(3);
     for (int i = 0; i < 3; i++) {
       executorService.execute(() -> {
-        while (inventory.addItem(new Item())) {};
+        while (inventory.addItem(new Item())) {
+          LOGGER.info("Adding another item");
+        }
       });
     }