folly::dynamic: use std::vector instead of folly::fbvector

folly::dynamic: use std::vector instead of folly::fbvector

Summary:
It appears that std::unordered_map is no longer relocatable in
gcc-4.7.  Use std::vector instead, until fbvector supports non-
relocatable types.

Test Plan: Built and ran facebar tests under gcc-4.7.  Also ran folly unittests.

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D543099