Reinstate CatBoost integraton test

[ablaom]

CatBoost 0.3.3 is now tagged and JuliaAI/CatBoost.jl#22 (comment) is closed.

Waiting for a fix:

• at Corner case: Training a classifier with only one class present CatBoost.jl#22

[ablaom]

Mmm. It seems integration tests still fail as the test log would otherwise report the flagged issue can be dropped. So there may be a new issue.