Keep NUnit at the same version between both projects

The SpecEasy project was at NUnit 2.6.2 and SpecEasy.Specs was at 2.6.3.  This required two version of Nunit to be restored by NuGet when only one is really needed.  I upgraded SpecEasy's `packages.config` to use version 2.6.3.

The target framework also was updated to 451 - would this be an issue?