When I was using a couple of shared projects, across different solutions, I fould it really hard to match the same version for all.
Until I discovered a very handy tool that comes with NuGet in Visual Studio (2013) menu:
Tools -> NuGet Package Manager -> Manage NuGet Packages for Solution ...
From here, to you install/uninstall package, add reference to a package for a list of projects. A real timesaver I didn't knew I had...