supporting iOS version

[dakeshi]

I think that we need to make clear about iOS version supporting issue again.
(related issue : #112, #113, related PRs: #170)

After 0.6.0, FirebaseUI supports iOS 8.0+.
I'm not sure that we need to mention 0.5 version which can support iOS 7.0.

And FirebaseUI.podspec use 7.0 for deployment_target but dev_podspec files support 8.0+.
I think that FirebaseUI.podspec needs to increase its version to 8.0.

[yorgosg]

Is iOS 7 no longer supported because of using Twitter login only?

[dakeshi]

@yorgosg, I think that there is more complicated reasons.

@mcdonamp mentioned this topic before in #75:

The root cause of all our CocoaPods issues is that we support iOS 7 (and thus, we ship static libs wrapped in Frameworks), while Apple (and CocoaPods by extension), really couldn't care less about supporting old OS's or framework developers. As further proof, I don't believe XCode 8 ships with iOS 7 support :)

Yes, he is right. Basically, Xcode 8 does not support iOS 7 deployment target anymore.
And dynamic framework support would be another reason, I think.

[morganchen12]

Our ability to adopt dynamic frameworks (which I would love to do) is limited by upstream Firebase only shipping static frameworks. It's unlikely that they'll change soon.

But yes, we should drop iOS 7 because TwitterKit (which we now have as a dependency) only supports iOS 8 and above, and we don't want to force external developers to keep multiple copies of Xcode around just to work on this codebase.

[morganchen12]

fixed in #176.