Switch to c++11

Summary:
Looks like gnu++11 is not very portable (see https://github.com/facebook/rocksdb/issues/80). We don't (yet) depend on any extensions of gnu++11, so I think we should switch back to c++11.

More on this topic here: http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Extensions.html

Test Plan: make check works :)

Reviewers: kailiu, dhruba, haobo

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16053