diff --git a/.travis.yml b/.travis.yml
index abf5bfe..5f3afd0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,6 +10,7 @@ addons:
     packages:
     - g++-4.8
     - gcc-4.8
+    sources:
     - ubuntu-toolchain-r-test
 
 install: