Support Flutter 0.6.0

[agrosner]

1. Updates dart dependency to go >=2.0.0 and <2.2.0. This is to enforce Dart2 in newer flutter versions.
2. Update version to 0.0.4.

This is related to:
#13
but that MR does not support Dart 2.1.0, required by Flutter 0.6.0.

[xqwzts]

Thanks for the PR @agrosner! I appreciate the contribution.

This has now been resolved by #17

I've bumped the sdk constraints to match the ones defined by the latest flutter beta.

A new release is published on pub with that as well: https://pub.dartlang.org/packages/flutter_circular_chart#-installing-tab- - you might need to run flutter packages upgrade to get it, or change your pubspec.yaml file to point to ^0.1.0 for this package.