message: Travis, add os: linux when using apt
diff: @@ -18,6 +18,7 @@ jobs:
  # - gitignore
  - stage: checks
  + os: linux
  name: "code-style"
  addons:
  apt:
  @@ -29,6 +30,7 @@ jobs:
  - ./runtest code-style
  - stage: checks
  + os: linux
  name: "documentation"
  addons:
  apt:
  @@ -44,6 +46,7 @@ jobs:
  - ./runtest documentation
  - stage: checks
  + os: linux
  name: "gitignore"
  addons:
  apt:
  