Add errorprone check and fix found errors

Adds gradle-errorprone-plugin 0.0.8, requires Gradle 2.6.

smack-tcp/src/test/java/org/jivesoftware/smack/tcp/PacketWriterTest.java

@@ -105,6 +105,7 @@ public class PacketWriterTest {
 
     public class BlockingStringWriter extends Writer {
         @Override
+        @SuppressWarnings("WaitNotInLoop")
         public void write(char[] cbuf, int off, int len) throws IOException {
             try {
                 wait();