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

vmx/rocksdb,Andymic/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,vmx/rocksdb,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,vmx/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb