Target .net 4.0 rather than 4.5

Adding the nuget package to a project using .net 4 causes a compile error with the message "The type or namespace name 'MailChimp' could not be found (are you missing a using directive or an assembly reference?)"

Building MailChimp.NET against .net 4.0 fixes the problem and there's nothing as yet in the library that needs .net 4.5

I did a very quick test referencing 4.0 mailchimp from a 4.5 project and it seems to work ok so this _shouldn't_ break anything for 4.5 projects either.