Fix for SSL validation on iOS 5.0 and 5.0.1

Incorporates a workaround provided by Apple in TN 2287:
https://developer.apple.com/library/ios/#technotes/tn2287/_index.html#//apple_ref/doc/uid/DTS40011309

Is basically the same change as this pull request:
https://github.com/pokeb/asi-http-request/pull/325

The main difference from the other pull request is rather than downgrading the SSL security across all iOS versions, I wrapped the SSL Level setting in an iOS version check.