Create Google-AdMob-Ads-SDK.podspec.json

I can't install this pod because version number is invalid.

https://github.com/CocoaPods/Specs/raw/master/Specs/Google-AdMob-Ads-SDK/6.12.0/Google-AdMob-Ads-SDK.podspec.json

googlemobileadssdkios.zip include source code version 6.12.2, but above podspec file specify version 6.12.0.