travis: add Go tip for testing

It would be good to test etcd with the latest Go branch,
so that we can find bugs either in Go or etcd in advance.

Reference:
https://docs.travis-ci.com/user/languages/go

.travis.yml

@@ -3,6 +3,7 @@ sudo: false
 go:
   - 1.4
   - 1.5
+  - tip
 
 addons:
   apt: