Update Travis to fix failing Linux build

Author: bengotow

.travis.yml

@@ -29,6 +29,7 @@ addons:
     packages:
       - execstack
       - fakeroot
+      - g++-8
       - rpm
 
 branches:
@@ -41,6 +42,7 @@ matrix:
   include:
     - os: linux
       dist: bionic
+      env: CC=gcc-8 CXX=g++-8
     - os: osx
       osx_image: xcode13.1
       env: CC=clang CXX=clang++ SIGN_BUILD=true