64 bit version for nuget

@darrenkopp

I've tried to change as little as possible, so I haven't bumped the version.

The packages are named libsassnet.x64 and libsassnet.Web.x64, but the dlls are still going to be libsassnet.dll and libsassnet.Web.dll 

I'm getting a warning when packing: "WARNING: Unable to extract metadata from 'libsassnet.dll'." but the nupkg seems to be ok.