ci: add gcc-9 to Travis CI build matrix

Change-Id: I719ef56d484ca6a743afe2ddd7486a2f43f59ad3

.travis.yml

@@ -17,6 +17,8 @@ matrix:
       env: COMPILER=g++-7
     - os: linux
       env: COMPILER=g++-8
+    - os: linux
+      env: COMPILER=g++-9
 
     # Linux/clang
     # https://apt.llvm.org/