Travis: make the exclude descriptions shorter

    ## Clang doesn't work in containerized builds, see below.

    - compiler: clang

      sudo: false

    ## We also exclude non-containerized gcc, because they're slow and redundant.

    ## Non-containerized gcc are slow and redundant.

    - compiler: gcc

      sudo: required

    ## And skip gcc on OSX, because the default compiler is clang

    ## gcc on OSX is less useful, because the default compiler is clang.

    - compiler: gcc

      os: osx