Only run Realm update checker if min required iOS version is >= iOS 7.0

This will allow users to include Realm in iOS app projects targeting iOS 6.x ([see user group discussion](https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/realm-cocoa/BPEsG53khI0/NsLuFplLukoJ)).

@tgoyne @alazier 
`[NSUserDefaults instancesRespondToSelector`