I just tried Optim.jl, but it dies because it uses @printf. I assume this is because it has not been updated to 1.0 yet.
I think Pkg installs all packages from the julia repository, regardless of whether they have been updated or not. In julia 2.0, the packages should declare this, too. alas, until then, does someone maintain and post a list of packages that have been updated to 1.0 (perhaps with version numbers)?
if a new user tries Julia, and encounters this problem, they will infer that Julia is not reliable, and not that this package is just not available.