travis: test with xcode8.3 (OS X 10.12)

Summary:
Use later xcode version from https://docs.travis-ci.com/user/osx-ci-environment
Closes https://github.com/facebook/rocksdb/pull/2128

Differential Revision: D4907471

Pulled By: yiwu-arbug

fbshipit-source-id: debf8e27baef71a5833c845401b1865bc75ac977

.travis.yml

@@ -4,6 +4,7 @@ language: cpp
 os:
   - linux
   - osx
+osx_image: xcode8.3
 jdk:
   - oraclejdk7
 cache: