Fix deployment target to match React Native (8.0)

[ndbroadbent]

I figured this out when the build was crashing on my simulator running iOS 8.4. With help from this comment on SO.

[alvaromb]

Hi @ndbroadbent!

Have you checked the implications of this change? Are the ads SDK supported for iOS 8.0?

[ndbroadbent]

Hi, I did test this on the iOS Simulator running iOS 8.4, and everything seemed to work fine.

I think the SDK supports iOS 7 and higher: https://firebase.google.com/docs/admob/ios/download

[philipheinser]

Also works for us with the target set to 8.0.

[alvaromb]

So, can we safely set the target to 8.0?

[alvaromb]

Ping @ndbroadbent @philipheinser

[ndbroadbent]

Hi, yes, should be totally safe to merge this. Apps cannot work on iOS 8 without this change.

[alvaromb]

Ok, @ndbroadbent, thanks for your quick feedback.

I'm in the middle of something, it's ok for you if we release this later this week?

[ndbroadbent]

Yes, no problem :) I am already just using a fork with my own changes, so I'm not in any hurry

[ndbroadbent]

@alvaromb sorry, just another ping on this. I just rebased to the latest commit on master