travis: run tests with Go 1.8.3

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:
-  - 1.8.1
+  - 1.8.3
   - tip
 
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