Try to fix travis by adding apt source

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 8f34664..751f4b9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,6 +9,7 @@
   apt:
     sources:
       - ubuntu-toolchain-r-test
+      - llvm-toolchain-xenial-8
     packages:
       - cmake
       - valgrind