Update dmlc-core. (#5466)

* Copy dmlc travis script to XGBoost.

tests/travis/travis_before_cache.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+# do nothing for now
+ls -alLR ${CACHE_PREFIX}

tests/travis/travis_setup_env.sh

@@ -0,0 +1,40 @@
+# script to be sourced in travis yml
+# setup all enviroment variables
+
+export CACHE_PREFIX=${HOME}/.cache/usr
+export PATH=${HOME}/.local/bin:${PATH}
+export PATH=${PATH}:${CACHE_PREFIX}/bin
+export CPLUS_INCLUDE_PATH=${CPLUS_INCLUDE_PATH}:${CACHE_PREFIX}/include
+export C_INCLUDE_PATH=${C_INCLUDE_PATH}:${CACHE_PREFIX}/include
+export LIBRARY_PATH=${LIBRARY_PATH}:${CACHE_PREFIX}/lib
+export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:${CACHE_PREFIX}/lib
+export DYLD_LIBRARY_PATH=${DYLD_LIBRARY_PATH}:${CACHE_PREFIX}/lib
+
+alias make="make -j4"
+
+# setup the cache prefix folder
+if [ ! -d ${HOME}/.cache ]; then
+    mkdir ${HOME}/.cache
+fi
+
+if [ ! -d ${CACHE_PREFIX} ]; then
+    mkdir ${CACHE_PREFIX}
+fi
+if [ ! -d ${CACHE_PREFIX}/include ]; then
+    mkdir ${CACHE_PREFIX}/include
+fi
+if [ ! -d ${CACHE_PREFIX}/lib ]; then
+    mkdir ${CACHE_PREFIX}/lib
+fi
+if [ ! -d ${CACHE_PREFIX}/bin ]; then
+    mkdir ${CACHE_PREFIX}/bin
+fi
+
+# setup CUDA path if NVCC_PREFIX exists
+if [ ! -z "$NVCC_PREFIX" ]; then
+    export PATH=${PATH}:${NVCC_PREFIX}/usr/local/cuda-7.5/bin
+    export CPLUS_INCLUDE_PATH=${CPLUS_INCLUDE_PATH}:${NVCC_PREFIX}/usr/local/cuda-7.5/include
+    export C_INCLUDE_PATH=${C_INCLUDE_PATH}:${NVCC_PREFIX}/usr/local/cuda-7.5/include
+    export LIBRARY_PATH=${LIBRARY_PATH}:${NVCC_PREFIX}/usr/local/cuda-7.5/lib64:${NVCC_PREFIX}/usr/lib/x86_64-linux-gnu
+    export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:${NVCC_PREFIX}/usr/local/cuda-7.5/lib64:${NVCC_PREFIX}/usr/lib/x86_64-linux-gnu
+fi