Fix BaseReferencedVersionBuilder's destructor order

Summary: BaseReferencedVersionBuilder now unreference version before destructing VersionBuilder, which is wrong. Fix it.

Test Plan:
make all check
valgrind test to tests that used to fail

Reviewers: igor, yhchiang, rven, ljin

Reviewed By: ljin

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D28101