message: Removed clang-4 from travis
  It's not going to work easily, I think it would work with libc++ though, but, let's just require clang-5.
diff: @@ -5,18 +5,16 @@ language: cpp
  env:
  - C_COMPILER=gcc-7 CXX_COMPILER=g++-7
  - - C_COMPILER=clang-4.0 CXX_COMPILER=clang++-4.0
  - C_COMPILER=clang-5.0 CXX_COMPILER=clang++-5.0
  before_install:
  - pyenv global system 3.5 # Workaround for travis-ci/issues/8363
  - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
  - wget -O - http://llvm.org/apt/llvm-snapshot.gpg.key | sudo apt-key add -
  - - echo "deb http://llvm.org/apt/trusty/ llvm-toolchain-trusty-4.0 main" | sudo tee -a /etc/apt/sources.list
  - echo "deb http://llvm.org/apt/trusty/ llvm-toolchain-trusty-5.0 main" | sudo tee -a /etc/apt/sources.list
  - sudo apt-get update -q
  - sudo apt-get install gcc-7 g++-7 -y
  - - sudo apt-get install clang-4.0 clang-5.0 -y
  + - sudo apt-get install clang-5.0 -y
  - sudo apt-get install libboost-all-dev libopencv-dev -y
  - wget https://cmake.org/files/v3.8/cmake-3.8.2-Linux-x86_64.tar.gz
  - tar -xvzf cmake-3.8.2-Linux-x86_64.tar.gz
  