Update MRProgress.podspec for AFNetworking

[AYastrebov]

• Fixed possible error when using AFNetworking pod with more recent version

[mrackwitz]

The pinned version for the UIKit subspec was intentionally. The code which belongs to this subspec makes some assumptions about the internals of the UIKit extension of AFNetworking. I'm not very happy about that fact, but taking the version out makes not only compiler errors, but even runtime errors very likely. But it's perhaps not obvious enough when integrating, that there is an implicit upper bound to AFNetworking by including this subspec in the Podfile. But in any case this must be at least updated.