message: Fix Travis after LLVM APT repo change
  After LLVM 6.0 was branched, the LLVM APT repositories changed so that
  clang-format-7 is in the "latest" repo instead of 6.0. We cannot use
  the LLVM-6.0 repo in Travis yet because it isn't in the repo whitelist.
  Update travis.yml to use clang-format-7
diff: @@ -22,7 +22,7 @@ addons:
  sources:
  # PPA for clang 5.0
  - llvm-toolchain-trusty-5.0
  - # PPA for clang-format 6.0
  + # PPA for clang-format 7
  - llvm-toolchain-trusty
  # PPA for a more recen libstdc++
  - ubuntu-toolchain-r-test
  @@ -38,7 +38,7 @@ addons:
  # Required dependency for GLFW on Linux
  - xorg-dev
  # Format using the latest and greatest
  - - clang-format-6.0
  + - clang-format-7
  before_install:
  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew update; fi
  @@ -70,4 +70,4 @@ script:
  - cd ..
  # Lint
  - - if [ "$TRAVIS_OS_NAME" == "linux" ]; then ./scripts/travis_lint_format.sh clang-format-6.0; fi
  + - if [ "$TRAVIS_OS_NAME" == "linux" ]; then ./scripts/travis_lint_format.sh clang-format-7; fi
  