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.

.travis.yml

@@ -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