* 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
* 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