Allow upgrade of JSON.jl compat

[LucasMatSP]

I'm working on a project that uses JSON.jl 1.3.0. And now I need to add packages that depend on OpenML.jl, but this package constraints JSON.jl to 0.21. I'd like to investigate the possibility of upgrading the compatibility requirement OpenML.jl has on JSON.jl.

[LucasMatSP]

Locally, upgrading the compat and running the tests, only the first one in the set below fails:

@testset "HTTP connection" begin
    @test typeof(response_test) <: Dict
    @test response_test["data_set_description"]["name"] == "iris"
    @test response_test["data_set_description"]["format"] == "ARFF"
end