Simplified instructions for CentOS
Simplified instructions for CentOS

Summary:
Minimal changes for improvements to INSTALL.md for CentOS/RHEL
Closes https://github.com/facebook/rocksdb/pull/2266

Differential Revision: D5053530

Pulled By: yiwu-arbug

fbshipit-source-id: c05f30299f8efec949b9001a73969d649536ea8a

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