travis: use Go 1.8.4

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:
-  - 1.8.3
+  - 1.8.4
   - tip
 
 notifications: