try GCC 8 on CI. GCC 7.3.0 seems to crash sometimes?

Author: kavon

.travis.yml

@@ -15,7 +15,7 @@ addons:
 
         packages:
             - ninja-build
-            - g++-7
+            - g++-8
             - llvm-6.0-dev
             - llvm-6.0-tools
             - clang-6.0
@@ -29,7 +29,7 @@ script:
   - ./xgboost/get.sh
   - mkdir _build && cd _build
   - git clone --depth=1 https://github.com/google/benchmark.git && git clone --depth=1 https://github.com/google/googletest.git benchmark/googletest && mkdir benchmark/_build && cd benchmark/_build && cmake .. -GNinja -DCMAKE_INSTALL_PREFIX=`pwd`/../_install && ninja && ninja install && cd ../..
-  - cmake -DLLVM_DIR=/usr/lib/llvm-6.0/cmake -DCMAKE_CXX_COMPILER=g++-7 -DCMAKE_C_COMPILER=gcc-7 -DEASY_JIT_EXAMPLE=ON -DEASY_JIT_BENCHMARK=ON -DBENCHMARK_DIR=`pwd`/benchmark/_install -DCMAKE_INSTALL_PREFIX=`pwd`/../_install .. -G Ninja
+  - cmake -DLLVM_DIR=/usr/lib/llvm-6.0/cmake -DCMAKE_CXX_COMPILER=g++-8 -DCMAKE_C_COMPILER=gcc-8 -DEASY_JIT_EXAMPLE=ON -DEASY_JIT_BENCHMARK=ON -DBENCHMARK_DIR=`pwd`/benchmark/_install -DCMAKE_INSTALL_PREFIX=`pwd`/../_install .. -G Ninja
   - ninja
   - ninja install
   - ninja check