[ios] reduce xcode version requirement to 7.0
[ios] reduce xcode version requirement to 7.0

Build bots only have 7.0.

flutter/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,turnidge/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,turnidge/flutter,Hixie/flutter,aghassemi/flutter,cbracken/flutter,flutter/flutter,turnidge/flutter,cbracken/flutter,aghassemi/flutter,flutter/flutter,turnidge/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,turnidge/flutter,tvolkert/flutter,tvolkert/flutter,aghassemi/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,aghassemi/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter