Fix evaluate API

[DhairyaLGandhi]

Addresses #10

[ghost]

This change breaks calling cross_validation function because of

Recommendation.jl/src/evaluation/cross_validation.jl

Line 38 in ca3bac9

accum += evaluate(recommender, truth_data, metric(), k)

I get

ERROR: LoadError: MethodError: no method matching evaluate(::MF, ::DataAccessor, ::MAE, ::Int64)

for

cross_validation(3, MAE, 15, MF, data, 5)