message: travis: add missing addons.apt.source
diff: @@ -16,6 +16,8 @@ matrix:
  env: CC=gcc-8 CXX=g++-8
  addons:
  apt:
  + sources:
  + - ubuntu-toolchain-r-test
  packages:
  - gcc-8
  @@ -24,6 +26,8 @@ matrix:
  env: CC=gcc-8 CXX=g++-8
  addons:
  apt:
  + sources:
  + - ubuntu-toolchain-r-test
  packages:
  - gcc-8
  @@ -32,6 +36,8 @@ matrix:
  env: CC=gcc-8 CXX=g++-8
  addons:
  apt:
  + sources:
  + - ubuntu-toolchain-r-test
  packages:
  - gcc-8
  @@ -40,6 +46,8 @@ matrix:
  env: CC=gcc-7 CXX=g++-7
  addons:
  apt:
  + sources:
  + - ubuntu-toolchain-r-test
  packages:
  - gcc-7
  - ninja-build
  