`bench` fails to build

[He Liu]

# github.com/dgraph-io/ristretto/bench
src/github.com/dgraph-io/ristretto/bench/bench.go:325:10: undefined: ristretto.PolicyLog

Found by gopkgs.io

[Karl McGuire]

Thanks for the heads up. We just overhauled the hit ratio tracking in Ristretto so I'm going to have to decouple bench and add my own tracking. Give me a few hours.

[Manish R Jain]

Sorry for the breakage. Did the change late night and was more focused on the performance and coverage. Thanks Karl for taking care of it.

[Karl McGuire]

Fixed in #35.