free builders in VersionSet::DumpManifest

Summary:
Reported by bootcamper
This causes ldb tool to fail the assertion in ~ColumnFamilyData()

Test Plan:
./ldb --db=/tmp/test_db1 --create_if_missing put a1 b1
./ldb manifest_dump --path=/tmp/test_db1/MANIFEST-000001

Reviewers: sdong, yhchiang, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29517