Fixed Windows Universal 8.1 project file for Release builds
Fixed Windows Universal 8.1 project file for Release builds

mganss/MailKit,nachocove/MailKit,jstedfast/MailKit,sstraus/MailKit,FubarDevelopment/MailKit,eino-makitalo/MailKit,rajeshwarn/MailKit,matt2003/MailKit,dsbenghe/MailKit,jamie-dainton/MailKit