Enable limits options in CcnxStackHelper

Slightly modifying the way of how forwarding strategy is hooked up
diff --git a/examples/ccnx-grid.cc b/examples/ccnx-grid.cc
index e60e962..c183413 100644
--- a/examples/ccnx-grid.cc
+++ b/examples/ccnx-grid.cc
@@ -87,7 +87,9 @@
 
   // Install CCNx stack
   NS_LOG_INFO ("Installing CCNx stack");
-  CcnxStackHelper ccnxHelper(Ccnx::NDN_FLOODING/*Ccnx::NDN_BESTROUTE*/);
+  CcnxStackHelper ccnxHelper;
+  ccnxHelper.SetForwardingStrategy ("ns3::CcnxFloodingStrategy");
+  ccnxHelper.EnableLimits (true);
   ccnxHelper.InstallAll ();
 
   // Install IP stack (necessary to populate FIB)