Travis test
diff --git a/.travis.yml b/.travis.yml
index 65fdb4a..9c733f5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,9 +8,11 @@
 language: cpp
 addons:
   apt:
+    sources:
+      - ubuntu-toolchain-r-test
     packages:
-      - gcc-7
-      - g++-7
+      - gcc-6
+      - g++-6
       - cmake