I think your problem is, that your T4 templates, which generate the entitties and the context are still in EF version 5.
First you have to delete the current code generation items, which are in the code behind of the model, namely <Modelname>.Context.tt and <Modelname>.tt.Next add a new EF version 6 code generator with Right click in the model designer-> Add Code Generation Item ... -> EF 6.x DbContext Generator.