Use Xenial repos

Author: TartanLlama

.travis.yml

@@ -73,7 +73,7 @@ matrix:
       addons:
         apt:
           sources:
-            - sourceline: "deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.9 main"
+            - sourceline: "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-3.9 main"
               key_url: "http://apt.llvm.org/llvm-snapshot.gpg.key"
             - ubuntu-toolchain-r-test
           packages: