.travis.yml: add gcc-9

Author: davidmalcolm

.travis.yml

@@ -64,6 +64,16 @@ matrix:
       env:
         - MATRIX_EVAL="CC=gcc-8 && CXX=g++-8"
 
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-9 gcc-9-plugin-dev
+      env:
+        - MATRIX_EVAL="CC=gcc-9 && CXX=g++-9"
+
 language: c
 compiler:
   - gcc