message: Move Travis image declarations to the build matrix
  This provides clearer feedback during Travis builds and allows for greater flexibility in the future.
diff: language: cpp
  -dist: xenial
  -osx_image: xcode10
  addons:
  apt:
  @@ -23,22 +21,29 @@ matrix:
  include:
  # Standard builds
  - os: linux
  + dist: xenial
  compiler: gcc
  - os: linux
  + dist: xenial
  compiler: clang
  - os: osx
  + osx_image: xcode10
  compiler: gcc
  - os: osx
  + osx_image: xcode10
  compiler: clang
  # Custom builds
  - os: linux
  + dist: xenial
  compiler: gcc
  env: MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
  - os: linux
  + dist: xenial
  compiler: gcc
  env: MATRIX_EVAL="CC=gcc-9 && CXX=g++-9 && PY=3"
  - os: linux
  + dist: xenial
  compiler: clang
  env: MATRIX_EVAL="CC=clang-8 && CXX=clang++-8 && PY=3"
  