EasyNetQ.DI: Changed all project Framework targets from 4.5 to 4.0

Turns out that not just the EasyNetQ.DI.Autofac project used 4.5 but also 
EasyNetQ.DI.Ninject 
EasyNetQ.DI.StructureMap
EasyNetQ.DI.Windsor, and
EasyNetQ.DI.Test

- Changed all project Framework targets from 4.5 to 4.0
- Updated Nuget References to 4.0 versions
- Updated NUnit package from 2.6.2 to 2.6.3