change DirectoryRule to FileRule in FileUnpacker
change DirectoryRule to FileRule in FileUnpacker

I think this should be a `FileRule` since `joinpath(s.dest, target)` is a file that should exist once the archive is unpacked.
andrewcooke/BinDeps.jl,chezou/BinDeps.jl,tkelman/BinDeps.jl,tlycken/BinDeps.jl,JuliaPackageMirrors/BinDeps.jl