[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

src/allreduce_robust.cc

@@ -50,12 +50,13 @@ void AllreduceRobust::Shutdown(void) {
   // execute check ack step, load happens here
   utils::Assert(RecoverExec(NULL, 0, ActionSummary::kCheckAck, ActionSummary::kSpecialOp),
                 "Shutdown: check ack must return true");
-#ifdef __APPLE__
-  // In OSX, one worker shutdowns and closes sockets while rest still run kCheckAck
-  // This cause rest workers checkandrecover and hang inf, https://github.com/dmlc/xgboost/pull/3818
+
+  // one worker shutdowns and closes sockets while rest still run kCheckAck,
+  // seems has something to do with time-wait state in tcp connection,
+  // this cause rest workers checkandrecover and hang inf,
+  // https://github.com/dmlc/xgboost/pull/3818
   // TODO(Chen Qin): a fundamental fix for this
-  sleep(2);
-#endif  // __APPLE__
+  sleep(1);
   AllreduceBase::Shutdown();
 }
 /*!