iOS podspec tweaks

[QuintinWillison]

At 72c575a this branch sorts out failing flutter run for iOS in my test project locally. The test project's pubspec.yaml looks like this (with the majority of comments removed for brevity):

name: ably_flutter_from_github
description: A new Flutter project.
publish_to: 'none' # Remove this line if you wish to publish to pub.dev
version: 1.0.0+1
environment:
  sdk: ">=2.7.0 <3.0.0"
dependencies:
  flutter:
    sdk: flutter
  ably_flutter_plugin:
    git:
      url: https://github.com/ably/ably-flutter.git
      ref: feature/ios-podspec-20200915
  cupertino_icons: ^0.1.3
dev_dependencies:
  flutter_test:
    sdk: flutter
flutter:
  uses-material-design: true

[tiholic]

It works like charm, now, on iOS!