Add sources for libstdc++6 >= 4.9

tautschnig · tautschnig · commit 3083dcdde371 · 2018-10-15T09:34:03.000Z
clang-format-6.0 requires a libstc++6 version newer than what Ubuntu 14.04 ships
by default; such a version is available from the ubuntu-toolchain-r-test PPA.
diff --git a/.travis.yml b/.travis.yml
@@ -10,6 +10,7 @@ jobs:
         apt:
           sources:
             - llvm-toolchain-trusty-6.0
+            - ubuntu-toolchain-r-test
           packages:
             - clang-format-6.0
       install:
