By default the xcode-plugin will sign the IPA when it runs xcrun on the app folder, dissabeling this will prevent xcrun from having the --sign option added to the command.
This fixes a known issue with Xcode on Mavericks:
http://stackoverflow.com/questions/32504355/error-itms-90339-this-bundle-is-invalid-the-info-plist-contains-an-invalid-ke http://stackoverflow.com/questions/32763288/ios-builds-ipa-creation-no-longer-works-from-the-command-line/32845990#32845990 http://cutting.io/posts/packaging-ios-apps-from-the-command-line/