Travis CI: Use Clang 9 on Linux

.travis.sh

@@ -32,6 +32,8 @@ set -e
 
 if [[ ${TRAVIS_OS_NAME} = osx ]]; then
     export PATH="/usr/local/opt/coreutils/libexec/gnubin${PATH:+:}${PATH}"
+elif [[ ${TRAVIS_OS_NAME} = linux ]]; then
+    export PATH="/usr/lib/llvm-9/bin:${PATH}"
 fi
 
 PS4='# '

.travis.yml

@@ -41,7 +41,11 @@ addons:
     update: true
     brewfile: true
   apt:
+    sources:
+      - llvm-toolchain-trusty
+      - ubuntu-toolchain-r-test
     packages:
+      - clang-9
       - cmake
       - docbook2x
       - dos2unix