message: travis: fix syntax of packages prop. in build matrix
diff: @@ -14,6 +14,8 @@ matrix:
  - name: "Python 2.7 gcc-8"
  python: 2.7
  env: CC=gcc-8 CXX=g++-8
  + addons:
  + apt:
  packages:
  - g++-8
  - gcc-8
  @@ -21,6 +23,8 @@ matrix:
  - name: "Python 3.5 gcc-8"
  python: 3.5
  env: CC=gcc-8 CXX=g++-8
  + addons:
  + apt:
  packages:
  - g++-8
  - gcc-8
  @@ -28,6 +32,8 @@ matrix:
  - name: "Python 3.7 gcc-8"
  python: 3.7
  env: CC=gcc-8 CXX=g++-8
  + addons:
  + apt:
  packages:
  - g++-8
  - gcc-8
  @@ -35,9 +41,12 @@ matrix:
  - name: "Python 3.7 gcc-7"
  python: 3.7
  env: CC=gcc-7 CXX=g++-7
  + addons:
  + apt:
  packages:
  - g++-7
  - gcc-7
  + - ninja-build
  addons:
  