Replace Ptr{None} with Ptr{Void} for 0.4-dev compatibility

Ptr{None} is deprecated, see https://github.com/JuliaLang/julia/issues/8423

Please check that this is correct, I just searched for usages in packages registrerd in METADATA, and might have made mistakes.
Merge pull request #20 from ivarne/patch-1

Replace Ptr{None} with Ptr{Void} for 0.4-dev compatibility