Use unpacked_dir option in BinDeps

This allows 32 and 64 bit installations to coexist on the same machine, like Jameson said (sorry for hijacking your mailing list thread with BinDeps stuff, this is a really useful library to have wrapped)

Windows build is green: https://ci.appveyor.com/project/tkelman/gumbo-jl/build/1.0.3