Microsoft has released the preview SDK assemblies for CRM 2015 Spring Update AKA 7.1 on Nuget. You have to filter on Preview releases to be able to see them.
If you are using Visual Studio 2015 check the box that says "Include Prerelease" and then from the Version list it will include the 7.1.0-preview. Visual Studio 2015 NuGet Package Manager is a little nicer as it lists the packages and then has a dropdown for version.
You can do the same thing in Visual Studio 2013
If you prefer to use NuGet Package Console you can use the following command
Install-Package Microsoft.CrmSdk.CoreAssemblies -Pre