[rabit harden] replace hardcopy dmlc-core headers with submodule links (#86)

* backport dmlc header changes to rabit

* use gitmodule to reference latest dmlc header files

* include ref to dmlc-core
fix cmake

* update cmake file, add cmake build traivs task

* try force using g++-4.8

* per feedback, update cmake

include/rabit/serializable.h

@@ -9,7 +9,7 @@
 #include <vector>
 #include <string>
 #include "./internal/utils.h"
-#include "../dmlc/io.h"
+#include "../../dmlc-core/include/dmlc/io.h"
 
 namespace rabit {
 /*!