Travis CI: Downgrade GCC to version 6

.travis.yml

@@ -27,13 +27,13 @@ before_install: |
     if [ "$TRAVIS_OS_NAME" = linux ]; then
       sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
       sudo apt-get -q update
-      sudo apt-get -qy install g++-7
+      sudo apt-get -qy install g++-6
       scripts/install_cmake.sh
       . scripts/install-cuda-ubuntu1604.sh
       pyenv global 3.6
       pip install --user requests gitpython
-      export CC=gcc-7
-      export CXX=g++-7
+      export CC=gcc-6
+      export CXX=g++-6
     elif [ "$TRAVIS_OS_NAME" == "osx" ]; then
       HOMEBREW_NO_AUTO_UPDATE=1 brew install -q python3
       pip3 install -q requests gitpython