Bump version to 4.2
Bump version to 4.2

Summary: Bump the version to 4.2 ( the unreleased version ), so that when fbcode_unittests run it can differentiate between old and new APIs

Test Plan: make check

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D49041

Vaisman/rocksdb,vmx/rocksdb,jalexanderqed/rocksdb,jalexanderqed/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,facebook/rocksdb,norton/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,tsheasha/rocksdb,jalexanderqed/rocksdb,flabby/rocksdb,ryneli/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,facebook/rocksdb,wenduo/rocksdb,norton/rocksdb,flabby/rocksdb,wenduo/rocksdb,flabby/rocksdb,OverlordQ/rocksdb,vmx/rocksdb,Andymic/rocksdb,norton/rocksdb,Vaisman/rocksdb,vmx/rocksdb,facebook/rocksdb,tsheasha/rocksdb,ryneli/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,hobinyoon/rocksdb,hobinyoon/rocksdb,wenduo/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,ryneli/rocksdb,Andymic/rocksdb,flabby/rocksdb,facebook/rocksdb,wenduo/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,tsheasha/rocksdb,vmx/rocksdb,flabby/rocksdb,tsheasha/rocksdb,wat-ze-hex/rocksdb,jalexanderqed/rocksdb,vmx/rocksdb,ryneli/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,facebook/rocksdb,wat-ze-hex/rocksdb,norton/rocksdb,OverlordQ/rocksdb,vmx/rocksdb,hobinyoon/rocksdb,bbiao/rocksdb,jalexanderqed/rocksdb,ryneli/rocksdb,norton/rocksdb,flabby/rocksdb,Vaisman/rocksdb,tsheasha/rocksdb,Vaisman/rocksdb,SunguckLee/RocksDB,facebook/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,vmx/rocksdb,ryneli/rocksdb,hobinyoon/rocksdb,hobinyoon/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,wenduo/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Vaisman/rocksdb,OverlordQ/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,norton/rocksdb,Vaisman/rocksdb,OverlordQ/rocksdb,tsheasha/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,Andymic/rocksdb,facebook/rocksdb,wenduo/rocksdb,Andymic/rocksdb,norton/rocksdb,hobinyoon/rocksdb,norton/rocksdb,Vaisman/rocksdb,bbiao/rocksdb,facebook/rocksdb,wenduo/rocksdb,OverlordQ/rocksdb,OverlordQ/rocksdb,ryneli/rocksdb