message: Travis: update to use clang 6.0, build on one thread
  Compiling on Linux Debug fails with what may be an out of memory for
  clang. Limit the number of jobs to try and avoid OOM.
diff: @@ -26,15 +26,15 @@ addons:
  # Everything under apt is only used on linux
  apt:
  sources:
  - # PPA for clang 5.0
  - - llvm-toolchain-trusty-5.0
  + # PPA for clang 6.0
  + - llvm-toolchain-trusty-6.0
  # PPA for clang-format 7
  - llvm-toolchain-trusty
  # PPA for a more recen libstdc++
  - ubuntu-toolchain-r-test
  packages:
  # Get recent compilers
  - - clang-5.0
  + - clang-6.0
  # Pull a recent version of libstdc++
  - libstdc++-6-dev
  # Ninja builds are faster
  @@ -56,14 +56,14 @@ install:
  script:
  # Use the more recent compilers we just installed
  - - if [ "$TRAVIS_OS_NAME" == "linux" ] && [ "$CXX" == "clang++" ]; then export CXX="clang++-5.0" CC="clang-5.0"; fi
  + - if [ "$TRAVIS_OS_NAME" == "linux" ] && [ "$CXX" == "clang++" ]; then export CXX="clang++-6.0" CC="clang-6.0"; fi
  # Build
  - mkdir -p build
  - cd build
  - cmake --version
  - cmake -G "Ninja" -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DDAWN_USE_WERROR=1 ..
  - - cmake --build .
  + - cmake --build . -- -j 1
  # Test
  - ./dawn_unittests
  