travis: update go versions

As discussed at https://github.com/Shopify/sarama/pull/950.

.travis.yml

@@ -1,7 +1,8 @@
 language: go
 go:
-- 1.7.3
-- 1.8
+- 1.7.x
+- 1.8.x
+- 1.9.x
 
 env:
   global: