I think that these are just beta releases of JuMP v0.19 in GitHub, but a new version has not been tagged at METADATA.jl yet, so the package manager does not know about it.
If you want to work with the cutting edge in JuMP, you could try Pkg.develop("JuMP") which will check out the master branch by default. You can then change the branch/tag later, if needed.
Maybe it also helps to just Pkg.add("MathOptInterface"), but that should be handled automatically, as a dependency (for JuMP). Of course, if you want to use MOI in your own code, you have to add it explicitly.