Summary:
Before this diff, there are two places with rocksdb versions. After the diff:
1. we only have one source of truth for rocksdb version
2. we have a script that we can use to get the version that we can use in other compilations (java, go, etc).
Test Plan: make
Reviewers: yhchiang, sdong, ljin
Reviewed By: ljin
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D24333