.travis: Use source whitelist alias for llvm-6 repository

Fixes issue with unauthenticated packages.
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewer: Aaron Watry

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@342337 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/.travis.yml b/.travis.yml
index a36c9cd..e1b558f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -64,7 +64,7 @@
       addons:
         apt:
           sources:
-            - sourceline: 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-6.0 main'
+            - llvm-toolchain-trusty-6.0
             - ubuntu-toolchain-r-test
           packages:
             - libedit-dev