This is a unfortunate result of Pkg choosing to downgrade JuMP instead of one of the solvers you have installed.
See https://github.com/JuliaOpt/GLPK.jl/issues/106#issuecomment-523659797
However. If you use Julia 1.2 at present, you will encounter the compiler hang mentioned above. The new version of JuMP will fix this, but until it’s released, we recommend you stick with Julia 1.1.