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

poison-pill/src/main/java/com/iluwatar/poison/pill/Message.java

@@ -66,7 +66,7 @@ public interface Message {
   /**
    * Enumeration of Type of Headers
    */
-  public enum Headers {
+  enum Headers {
     DATE, SENDER
   }