Adjust a source line in .travis.yml

Author: Roberto Di Remigio

.travis.yml

@@ -87,7 +87,7 @@ matrix:
     addons: &4
       apt:
         sources:
-        - llvm-toolchain-precise
+        - llvm-toolchain-precise-3.8
         - ubuntu-toolchain-r-test
         - boost-latest
         - george-edison55-precise-backports