ci/travis: update xcode version to 8.3

Xcode version: 8.3.3
macOS version: 10.12

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

Author: zchee

.travis.yml

@@ -1,5 +1,5 @@
 os: osx
-osx_image: xcode8
+osx_image: xcode8.3
 
 language: go
 go:
@@ -15,6 +15,10 @@ env:
     - OPAMYES=1
     - V=1
 
+cache:
+  directories:
+    - $HOME/Library/Caches/Homebrew
+
 before_install:
   - uname -a
   - env | sort