Tentative fix Clang 4.0 automated tests

.travis.yml

@@ -35,8 +35,7 @@ matrix:
       addons:
         apt:
           sources:
-            - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-4.0
+            - llvm-toolchain-trusty-4.0
           packages:
             - clang-4.0
       env: COMPILER=clang++-4.0