* Ensure that configured header (build_config.h) from dmlc-core is picked up by Rabit and XGBoost
* Check which Rabit target is being used
* Use CMake 3.13 in all Jenkins tests
* Upgrade CMake in Travis CI
* Install CMake using Kitware installer
* Remove existing CMake (3.12.4)
* Use devtoolset-6.
* [CI] Use devtoolset-6 because devtoolset-4 is EOL and no longer available
* CUDA 9.0 doesn't work with devtoolset-6; use devtoolset-4 for GPU build only
Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>
* All Linux tests are now in Jenkins CI
* Tests are now de-coupled from builds. We can now build XGBoost with one version of CUDA/JDK and test it with another version of CUDA/JDK
* Builds (compilation) are significantly faster because 1) They use C5 instances with faster CPU cores; and 2) build environment setup is cached using Docker containers
