compilation bug with gorocksdb

[cv711]

github.com/blevesearch/blevex/rocksdb/reader.go:58:12: r.snapshot.Release undefined (type *gorocksdb.Snapshot has no field or method Release)

[owbone]

This seems like just a breaking API change in github.com/tecbot/gorocksdb. We work around it by vendoring it at 209fbe7 (the parent commit of the breaking change).