Updating travis config to add libstdc++6 dependency

.travis.yml

@@ -4,6 +4,9 @@ go:
   - 1.8
   - tip
 before_install:
+  - sudo add-apt-repository ppa:ubuntu-toolchain-r/test
+  - sudo apt-get update
+  - sudo apt-get install libstdc++6
   - go get github.com/mattn/goveralls
   - ./contrib/grab-headless_shell.sh $HOME/hs
 script: