Fixed typo in `#if TARGET_OS_IPHONE`
`currentSystemVersion` was formerly reporting "Version 7.1.1 (Build 11D201)", now with the fix it reports "7.1.1".
Fixed issue with empty email address being saved as Person's email when sent from Engagement Rating Flow's Feedback Dialog.
CC https://github.com/apptentive/apptentive-ios/commit/70b80d56fbedd66d6ee47894ac19af3aadb9d4e5

Causes issue when saved into NSUserDefaults:
https://github.com/apptentive/apptentive-ios/commit/04062126dd04648f11b11f9d10f67671aa662a95#diff-92924b5b3b8a9efa9bef7cae889827cbR155
cleanup
Email is set to `[NSNull null]` in `apiJSON` to delete email from server. But `[NSNull null]` crashes if saved in NSUserDefaults.
Changed ATPersonUpdater to save a version of the person `apiJSON` that does not contain `[NSNull null` email.
Fixes IOS-526
Merge pull request #99 from apptentive/Next

Fix for saving `[NSNull null]` email to NSUserDefaults