Fix travis
Fix travis

Summary: Fix travis for mac by using gflags package instead of doing `brew install`

Test Plan: https://travis-ci.org/facebook/rocksdb/builds/156358515

Reviewers: sdong, andrewkr, lightmark, yiwu

Reviewed By: yiwu

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D62997

facebook/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,wenduo/rocksdb,Andymic/rocksdb,norton/rocksdb,wat-ze-hex/rocksdb,norton/rocksdb,norton/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,tsheasha/rocksdb,facebook/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,wenduo/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,OverlordQ/rocksdb,norton/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,tsheasha/rocksdb,wenduo/rocksdb,OverlordQ/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,bbiao/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,tsheasha/rocksdb,vmx/rocksdb,facebook/rocksdb,OverlordQ/rocksdb,wenduo/rocksdb,OverlordQ/rocksdb,SunguckLee/RocksDB,tsheasha/rocksdb,wat-ze-hex/rocksdb,facebook/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,wenduo/rocksdb,norton/rocksdb,tsheasha/rocksdb,norton/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,facebook/rocksdb,tsheasha/rocksdb,vmx/rocksdb,facebook/rocksdb,norton/rocksdb,Andymic/rocksdb,norton/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,OverlordQ/rocksdb,vmx/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,facebook/rocksdb,vmx/rocksdb,OverlordQ/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,tsheasha/rocksdb,bbiao/rocksdb,vmx/rocksdb,facebook/rocksdb,bbiao/rocksdb,wenduo/rocksdb