#180 : use `PostgreSqlVersion.ToString()`

[Ledragon]

Hi, Thanks for the project, it is awesome. As we are still stuck with EF6, I stumbled upon the same problem as in issue #180 while using Postgres in a docker container hosted in a debian VM. Here is a proposed fix for that problem.

I know the project is discouraged, but I think I might not be the only one stuck with EF6 at the moment; I let you judge if that can be integrated and deployed or not.

Kind regards, Hugues