Double the timeout to 120s in MemoryLeakTestUtil

Because an failing test on Travis was observered using a timeout of
60s:

org.jivesoftware.smackx.muc.MucMemoryLeakTest > mucMemoryLeakTest FAILED
    java.lang.AssertionError: No reference found after 60000ms
        at org.junit.Assert.fail(Assert.java:88)
        at org.junit.Assert.assertTrue(Assert.java:41)
        at org.junit.Assert.assertNotNull(Assert.java:712)
        at org.jivesoftware.smack.util.MemoryLeakTestUtil.assertReferencesQueueSize(MemoryLeakTestUtil.java:110)
        at org.jivesoftware.smack.util.MemoryLeakTestUtil.noResourceLeakTest(MemoryLeakTestUtil.java:103)
        at org.jivesoftware.smackx.muc.MucMemoryLeakTest.mucMemoryLeakTest(MucMemoryLeakTest.java:29)

smack-core/src/test/java/org/jivesoftware/smack/util/MemoryLeakTestUtil.java

@@ -104,7 +104,7 @@ public static <M extends Manager> void noResourceLeakTest(Function<DummyConnecti
     }
 
     private static void assertReferencesQueueSize(ReferenceQueue<?> referenceQueue, int expectedSize) throws IllegalArgumentException, InterruptedException {
-        final int timeout = 60000;
+        final int timeout = 120000;
         for (int itemsRemoved = 0; itemsRemoved < expectedSize; ++itemsRemoved) {
             Reference<?> reference = referenceQueue.remove(timeout);
             assertNotNull("No reference found after " + timeout + "ms", reference);