update test version.

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

.travis.yml

@@ -2,9 +2,9 @@ language: go
 sudo: false
 
 go:
-  - 1.5.4
-  - 1.6.4
-  - 1.7.4
+  - 1.6.x
+  - 1.7.x
+  - 1.8.x
   - tip
 
 script: