Don't use deprecated Futures#addCallback method (#78)

## Before this PR
tracing-java doesn't work with guava 27

## After this PR
tracing-java works with guava 27

tracing/src/main/java/com/palantir/tracing/AsyncSpanObserver.java

@@ -73,7 +73,7 @@ public void onFailure(Throwable error) {
                     log.trace("Failed to notify observer", error);
                     numInflights.decrementAndGet();
                 }
-            });
+            }, MoreExecutors.directExecutor());
         } else {
             log.trace("Failed to notify span observer since the maximum number of allowed concurrent observations was "
                     + "exceeded", SafeArg.of("maxInflights", maxInflights));