message: Use make -j2 builds on Travis CI
  This should save ~5 seconds for Linux/macOS builds by parallelizing C++
  compilation for each target.
diff: @@ -18,17 +18,17 @@ matrix:
  - TARGET="Visual Studio 15 2017 Win64"
  script:
  - - if [[ "$TRAVIS_COMPILER" == "gcc" ]]; then make config=coverage test; fi
  - - if [[ "$TRAVIS_COMPILER" == "clang" ]]; then make config=sanitize test; fi
  - - if [[ "$TRAVIS_OS_NAME" != "windows" ]]; then make config=debug test; fi
  - - if [[ "$TRAVIS_OS_NAME" != "windows" ]]; then make config=release test; fi
  - - if [[ "$TRAVIS_OS_NAME" != "windows" ]]; then make config=release gltfpack; fi
  + - if [[ "$TRAVIS_COMPILER" == "gcc" ]]; then make -j2 config=coverage test; fi
  + - if [[ "$TRAVIS_COMPILER" == "clang" ]]; then make -j2 config=sanitize test; fi
  + - if [[ "$TRAVIS_OS_NAME" != "windows" ]]; then make -j2 config=debug test; fi
  + - if [[ "$TRAVIS_OS_NAME" != "windows" ]]; then make -j2 config=release test; fi
  + - if [[ "$TRAVIS_OS_NAME" != "windows" ]]; then make -j2 config=release gltfpack; fi
  - if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then cmake -G "$TARGET" -DBUILD_DEMO=ON -DBUILD_TOOLS=ON; fi
  - if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then cmake --build . -- -property:Configuration=Debug -verbosity:minimal; fi
  - if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then ./Debug/demo.exe demo/pirate.obj; fi
  - if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then cmake --build . -- -property:Configuration=Release -verbosity:minimal; fi
  - if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then ./Release/demo.exe demo/pirate.obj; fi
  - - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then make config=iphone; fi
  + - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then make -j2 config=iphone; fi
  after_script:
  - if [[ "$TRAVIS_COMPILER" == "gcc" ]]; then
  