Adjust to the new version of goleveldb.

(And yes, we do want vendoring for that... This is just the quick fix.)

Change-Id: I9d347a64d96de6b3390a0e35c8d466f14bb83e4e
Merge pull request #423 from prometheus/u-c-l/adjust-to-new-goleveldb

Adjust to the new version of goleveldb.