Remove snappy from RocksDB distribution

Summary:
Argumentation here: https://github.com/facebook/rocksdb/issues/9

Even though we include snappy in the distribution, we do not link with it if we don't have snappy installed on the system.

Installing snappy is easy nowadays, just type:
sudo apt-get install libsnappy-dev

Test Plan: compile on ubuntu

Reviewers: dhruba, kailiu

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14133