Update Google-Maps-iOS-SDK.podspec 1.4.0 with the right deployment target

The iOS deployment target is wrong.
Google says: "While the iOS 6.0 SDK is required for development, the Google Maps SDK will work on iOS 5.1 and above.".
https://developers.google.com/maps/documentation/ios/intro?hl=fr#supported_platforms
Merge pull request #3282 from pyby/master

Update Google-Maps-iOS-SDK.podspec 1.4.0 with the right deployment target