Summary:
Given that we have cut 5.15, we should bump the version number to the next
version, i.e. 5.16.
Also update HISTORY.md
cc sagar0
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4176

Differential Revision: D8977965

Pulled By: riversand963

fbshipit-source-id: 481d75d2f446946f0eb2afb7e94ef894c8c87e1e