Bump guava version to 18.0

New version fixes 16 bugs and includes some API changes [1].
MoreExecutors.directExecutor() is now the new lightweight version
of now deprecated sameThreadExecutor().

[1] https://code.google.com/p/guava-libraries/wiki/Release18

Change-Id: I98dbf951b82095504272cc90f1468652621ae2bd

gerrit-pgm/src/main/java/com/google/gerrit/pgm/RebuildNotedb.java

@@ -130,7 +130,7 @@ public class RebuildNotedb extends SiteProgram {
             futures.add(future);
             future.addListener(
                 new RebuildListener(c.getId(), future, ok, doneTask, failedTask),
-                MoreExecutors.sameThreadExecutor());
+                MoreExecutors.directExecutor());
           }
 
           mpm.waitFor(Futures.transform(Futures.successfulAsList(futures),
@@ -212,7 +212,7 @@ public class RebuildNotedb extends SiteProgram {
           dbInjector.getInstance(WorkQueue.class)
             .createQueue(threads, "RebuildChange"));
     } else {
-      return MoreExecutors.sameThreadExecutor();
+      return MoreExecutors.newDirectExecutorService();
     }
   }