message: Update Travis build settings
  This changelist updates Travis settings to accommodate recent changes to their build environments.
diff: @@ -30,20 +30,20 @@ matrix:
  # Custom builds
  - os: linux
  - dist: xenial
  + dist: bionic
  compiler: gcc
  env: MATRIX_EVAL="CC=gcc-6 && CXX=g++-6 && PY=2"
  addons:
  apt:
  - sources: ubuntu-toolchain-r-test
  packages: [*packages_base, g++-6]
  - os: linux
  - dist: xenial
  + dist: bionic
  compiler: gcc
  env: MATRIX_EVAL="CC=gcc-9 && CXX=g++-9"
  addons:
  apt:
  - sources: ubuntu-toolchain-r-test
  + sources:
  + sourceline: "ppa:ubuntu-toolchain-r/test"
  packages: [*packages_base, g++-9]
  - os: linux
  dist: bionic
  