added xvfb, as the package needs Gtk, which needs a display system, although we don't open windows in the first place.
added xvfb, as the package needs Gtk, which needs a display system, although we don't open windows in the first place.

IainNZ/PackageEvaluator.jl,IainNZ/PackageEvaluator.jl,JuliaCI/PackageEvaluator.jl,JuliaCI/PackageEvaluator.jl,JuliaCI/PackageEvaluator.jl,ScottPJones/PackageEvaluator.jl,ScottPJones/PackageEvaluator.jl,IainNZ/PackageEvaluator.jl,ScottPJones/PackageEvaluator.jl