Merge pull request #3923 from zhaowenlan1779/fix-clang-ci

Fix clang-format CI

.travis.yml

@@ -7,9 +7,8 @@ matrix:
       addons:
         apt:
           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'
+            - llvm-toolchain-trusty-6.0
+            - ubuntu-toolchain-r-test
           packages:
             - clang-format-6.0
       script: "./.travis/clang-format/script.sh"