Travis: Update style checks to clang-format 6.0.1

.travis.yml

@@ -44,7 +44,7 @@ addons:
   apt:
     sources:
       - ubuntu-toolchain-r-test
-      - llvm-toolchain-trusty-5.0
+      - llvm-toolchain-trusty-6.0
     packages:
       - build-essential
       - scons
@@ -62,6 +62,7 @@ addons:
 
       # For style checks.
       - clang-format-5.0
+      - libstdc++6 # >= 4.9 needed for clang-format-6.0
 
 before_install:
   - if [ "$STATIC_CHECKS" = "yes" ]; then

misc/travis/clang-format.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-CLANG_FORMAT=clang-format-5.0
+CLANG_FORMAT=clang-format-6.0
 
 if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
     # Check the whole commit range against $TRAVIS_BRANCH, the base merge branch