Converting travis apt dependencies to official addons config style

.travis.yml

@@ -3,10 +3,13 @@ go:
   - 1.7
   - 1.8
   - tip
+addons:
+  apt:
+    sources:
+      - ubuntu-toolchain-r/test
+    packages:
+      - libstdc++6
 before_install:
-  - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
-  - sudo apt-get update
-  - sudo apt-get install -y libstdc++6
   - go get github.com/mattn/goveralls
   - ./contrib/grab-headless_shell.sh $HOME/hs
 script: