`as_df=false` requires `using DataFrames`

[pedromxavier]

When running

julia> using MLDatasets: Iris

julia> X, y = Iris(as_df=false)[:]

on julia 1.8.2, MLDatasets v0.7.5 the user is prompted with an error asking for DataFrames to be installed.

[CarloLucibello]

That is intended behavior since in the constructor we do some intermediate manipulations using dataframes for convenience, but with a little additional effort that can be avoided (fix in #185)