I'm very surprised. I searched for system requirements for Microsoft OWIN aka Katana. I had a hard time finding a definitive answer on this as well.
So when in doubt, head to the source:
According to the project file:
<TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
That is why NuGet won't let you install the package. You need to update your project to .NET 4.5 Framework.