[UPDATE] Update rabit and threadlocal (#2114)

* [UPDATE] Update rabit and threadlocal

* minor fix to make build system happy

* upgrade requirement to g++4.8

* upgrade dmlc-core

* update travis

tests/travis/run_test.sh

@@ -21,6 +21,9 @@ if [ ${TRAVIS_OS_NAME} == "osx" ]; then
     echo 'USE_OPENMP=0' >> config.mk
     echo 'TMPVAR := $(XGB_PLUGINS)' >> config.mk
     echo 'XGB_PLUGINS = $(filter-out plugin/lz4/plugin.mk, $(TMPVAR))' >> config.mk
+else
+    # use g++-4.8 for linux
+    export CXX=g++-4.8
 fi
 
 if [ ${TASK} == "python_test" ]; then