Bump "Error Prone" to 2.0.15

and fix a few things :)

smack-extensions/src/main/java/org/jivesoftware/smackx/pep/PEPManager.java

@@ -91,6 +91,7 @@ public final class PEPManager extends Manager {
     private PEPManager(XMPPConnection connection) {
         super(connection);
         StanzaListener packetListener = new StanzaListener() {
+            @Override
             public void processStanza(Stanza stanza) {
                 Message message = (Message) stanza;
                 EventElement event = EventElement.from(stanza);