update golang testing to 1.6.4 and 1.7.4

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

.travis.yml

@@ -3,8 +3,8 @@ sudo: false
 
 go:
   - 1.5.4
-  - 1.6.3
-  - 1.7.3
+  - 1.6.4
+  - 1.7.4
   - tip
 
 script: