Summary:
Rocksdb currently has many references to std::map.emplace_back()
which is not implemented in gcc 4.7, but valid in gcc 4.8. Confirmed that
it did not build with gcc 4.7, but builds fine with gcc 4.8
Closes https://github.com/facebook/rocksdb/pull/1272

Differential Revision: D4101385

Pulled By: IslamAbdelRahman

fbshipit-source-id: f6af453