travis: specify Go minor versions

1.9.x doesn't work with travis Go 'gimme'.
https://travis-ci.org/coreos/etcd/jobs/283789582#L616-L629

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

.travis.yml

@@ -4,7 +4,7 @@ go_import_path: github.com/coreos/etcd
 sudo: false
 
 go:
-  - 1.9.x
+  - 1.9.1
   - tip
 
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