Add scikit-learn dependency to macrel

[luispedro]

New version uses scikit-learn instead of r-randomForest, so update to
that.

• This PR updates an existing recipe.

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[luispedro]

Original bug report: BigDataBiology/macrel#17

[luispedro]

@BiocondaBot please add label

[dpryan79]

@bioconda-bot please fetch artifacts

[BiocondaBot]

Package(s) built on CircleCI are ready for inspection:

Arch	Package	Repodata
linux-64	macrel-0.3.1-py27h516909a_1.tar.bz2	repodata.json
linux-64	macrel-0.3.1-py36h516909a_1.tar.bz2	repodata.json
linux-64	macrel-0.3.1-py37h516909a_1.tar.bz2	repodata.json

---

You may also use conda to install these:

• For packages on linux-64:

conda install -c https://92347-42372094-gh.circle-artifacts.com/0/tmp/artifacts/packages <package name>

---

Docker image(s) built:

Package	Tag	Install with docker
macrel	0.3.1--py27h516909a_1	show curl "https://92347-42372094-gh.circle-artifacts.com/0/tmp/artifacts/images/macrel%3A0.3.1--py27h516909a_1.tar.gz" | gzip -dc | docker load
macrel	0.3.1--py36h516909a_1	show curl "https://92347-42372094-gh.circle-artifacts.com/0/tmp/artifacts/images/macrel%3A0.3.1--py36h516909a_1.tar.gz" | gzip -dc | docker load
macrel	0.3.1--py37h516909a_1	show curl "https://92347-42372094-gh.circle-artifacts.com/0/tmp/artifacts/images/macrel%3A0.3.1--py37h516909a_1.tar.gz" | gzip -dc | docker load

[dpryan79]

@bioconda-bot please merge

[BiocondaBot]

I will attempt to upload artifacts and merge this PR. This may take some time, please have patience.