DI.Autofac: Changed Target framework to .NET Framework 4 Client Profile

**Fixes the following compilation error:**
The type or namespace name 'DI' does not exist in the namespace 'EasyNetQ' (are you missing an assembly reference?)

**and the following warning:**
The primary reference "EasyNetQ.DI.Autofac, Version=0.28.4.240, Culture=neutral, processorArchitecture=MSIL" could not be resolved because it was built against the ".NETFramework,Version=v4.5" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.0".