Hard deps and run docs only on #master

[tlienart]

• cleans up duplicate and inconsistent import and using statements
• removes requires and optional deps and clean up project.toml as a result
• fixes travis to only test docs on master

all runs, there's one thing that fails but seems unrelated (I had it when testing before changing anything)

M = scitype(rms)
@test_broken M().prediction_type == :deterministic

I suggest we merge this and figure out the prediction type issue in another PR as it's a minor issue.

This should be merged after JuliaAI/ScientificTypes.jl#38 (1) and JuliaAI/MLJBase.jl#78 (2) are merged (particularly (2) as it depends upon it for the @load_*)

[tlienart]

That should be ready to merge (after the MLJBase one), also @ablaom see how https://travis-ci.com/alan-turing-institute/MLJ.jl/builds/135124126 does not test the docs FYI

[ablaom]

I'm happy with this and it looks like tests are going to pass. Will merge after tests and JuliaAI/MLJBase.jl#78