[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

scripts/travis_script.sh

@@ -25,3 +25,9 @@ if [ ${TASK} == "test" ]; then
     ../scripts/travis_runtest.sh || exit -1
 fi
 
+if [ ${TASK} == "cmake-build" ]; then
+    mkdir build
+    cd build
+    cmake ..
+    make all || exit -1
+fi