Travis: Add missing PPA for newer libstdc++

.travis.yml

@@ -9,6 +9,7 @@ matrix:
           sources:
             - sourceline: 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-6.0 main'
               key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
+            - sourceline: 'deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu trusty main'
           packages:
             - clang-format-6.0
       script: "./.travis/clang-format/script.sh"