Add MIXPANEL_NO_IFA to older version of lib

Apple's app review team may reject apps that use the AdSupport framework's advertisingIdentifier (IFA) but do not show ads. The MIXPANEL_NO_IFA preprocessor macro—you can set it in your target's XCode build settings—will strip IFA usage out of the Mixpanel lib. Instead of IFA, we'll use UIDevice's identifierForVendor as the default Mixpanel distinct ID.

This functionality is already in the most recent versions of the lib. However, apps that still want to support iOS 5 must use the 2.0.x releases. This release makes MIXPANEL_NO_IFA available to them.