message: construction_worker: removing Clang 3.9 and 4.0
  These versions are still not working properly:
diff: @@ -183,17 +183,17 @@ matrix:
  - os: linux
  compiler: gcc
  - env:
  - - COMPILER=g++-6
  - - CXXFLAGS=-std=c++1z
  + env: COMPILER=g++-7
  addons:
  apt:
  sources: ['ubuntu-toolchain-r-test']
  - packages: g++-6
  + packages: g++-7
  - os: linux
  compiler: gcc
  - env: COMPILER=g++-7
  + env:
  + - COMPILER=g++-7
  + - CXXFLAGS=-std=c++17
  addons:
  apt:
  sources: ['ubuntu-toolchain-r-test']
  @@ -229,22 +229,6 @@ matrix:
  env: LLVM_VERSION=3.8.1
  compiler: clang
  - - os: linux
  - compiler: clang
  - env: COMPILER=clang++-3.9
  - addons:
  - apt:
  - sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-3.9']
  - packages: clang-3.9
  -
  - - os: linux
  - compiler: clang
  - env: COMPILER=clang++-4.0
  - addons:
  - apt:
  - sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-4.0']
  - packages: clang-4.0
  -
  # - os: linux
  # addons:
  # apt:
  