Support NuGet install for .NET 3.5 projects

Got this error when installing the latest OctoPack for a .NET 3.5 project.

>Could not install package 'OctoPack 3.0.20'. You are trying to install this package into a project that targets '.NETFramework,Version=v3.5', but the package does not contain any assembly references or content files that are compatible with tha
t framework. For more information, contact the package author.

Version 2 supported projects targeting .NET 3.5 and I didn't see any documentation saying support had been dropped so I tested this quick fix to get it installed.