ci: add gcc-9 to Travis CI build matrix

Change-Id: Id4165765e2099ba9ff242dcef6b4a6fe08eaba97
diff --git a/.travis.yml b/.travis.yml
index 47579e9..136ded4 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -17,6 +17,8 @@
       env: COMPILER=g++-7
     - os: linux
       env: COMPILER=g++-8
+    - os: linux
+      env: COMPILER=g++-9
 
     # Linux/clang
     # https://apt.llvm.org/