ci/travis: update go version to 1.8 and disable verbose build

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>

Author: zchee

.travis.yml

@@ -5,15 +5,15 @@ osx_image: xcode8
 
 language: go
 go:
-  - 1.7.3
+  - 1.8
   - tip
 
 
 env:
   global:
     - GO_BUILD_TAGS='lib9p qcow2'
     - HOMEBREW_NO_AUTO_UPDATE=1
-    - MAKEFLAGS="V=1 -j$(sysctl -n hw.ncpu)"
+    - MAKEFLAGS="-j$(sysctl -n hw.ncpu)"
     - RELEASE_GO_VERSION=''