Use pubspec version on each platform Library version

[danielgomezrico]

Current

We need to go to these files manually and put the same value from pubspec.yml (https://github.com/la-haus/flutter-segment/blob/master/pubspec.yaml#L3) in here on each release

Each file where it should be updated:

• https://github.com/la-haus/flutter-segment/blob/master/ios/flutter_segment.podspec#L6
• https://github.com/la-haus/flutter-segment/blob/master/android/build.gradle#L2

Expected

We just have to update pubspec.yml version and each platform will use that version somehow without a manual update