Mixed mode assembly is built against version ‘v2.0.50727′ of the runtime

Try to use this exact startup tag in your app.config under configuration node

<startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
    <requiredRuntime version="v4.0.20506" />
  </startup>