Switch CMake CI build to GCC 9

Author: d-frey

.travis.yml

@@ -251,9 +251,9 @@ matrix:
       addons:
         apt:
           sources: ['ubuntu-toolchain-r-test']
-          packages: ['g++-8']
+          packages: ['g++-9']
       env:
-        - CXX=g++-8
+        - CXX=g++-9
       script:
         - $CXX --version
         - mkdir build