Add GCC 9.x to the Travis CI matrix.

Change-Id: I5f7e3ea5fdd23ff181f8d1037e86f7578d21ab0b
Reviewed-on: https://code-review.googlesource.com/c/re2/+/41350
Reviewed-by: Paul Wankadia <junyer@google.com>

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 0ddc4a9..f89c96d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -61,6 +61,15 @@
             - g++-8
       env:
         - MATRIX_EVAL="CC=gcc-8 CXX=g++-8"
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-9
+      env:
+        - MATRIX_EVAL="CC=gcc-9 CXX=g++-9"
 
     - os: linux
       addons: