Travis: Change go version used to 1.9

So that tests work (we are using t.Helper now).

.travis.yml

@@ -2,7 +2,7 @@ dist: trusty
 sudo: false
 language: go
 go:
-- 1.8.1
+- 1.9
 
 install:
 # This script is used by the Travis build to install a cookie for