Recheck readme example after MLJ.jl update

[ablaom]

In the pending PR #10, the example does using MLJBase, MLJModels, and so forth. After we rollout the new version of MLJ that will include using MLJFlow we can simplify the example to do using MLJ and so forth.

[ablaom]

? We haven't rolled out changes to MLJ.jl yet. We first need the new MLJBase.jl release, and then add using MLJFlow to MLJ/src/MLJ.jl`.

[ablaom]

@pebeto I suggest reverting the readme example so that it actually works with the current MLJFlow#dev and your MLJBase PR.

[ablaom]

Also, I suggest you state in the example "Requires MLJ.jl version 0.19.3 or higher."

[pebeto]

Now on dev. Waiting until the next release to move it to master.