Update dependencies

[Zazo032]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

This rolls minimum Dart SDK to the bundled with Flutter's latest stable version. Also, bumps pedantic and octo_image to latest versions. The update of octo_image is required for placeholders to not be wrongly placed.

💥 Does this PR introduce a breaking change?

From octo_image:

[0.3.0] - 2020-09-07 
Fixed bug when fitting placeholders or error widgets.
BREAKING: Doesn't automatically center placeholders and error widgets anymore.

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[renefloor]

Thanks, I just wanted to update the dependencies.
IMO it is not needed to increase the minimum version of Flutter to 1.20.3, for this package 1.20.0 is just fine right?
Where can you find the dart versions shipped with the flutter sdk?

[Zazo032]

It's OK if 1.20.0 is the minimum version, there are 3 hotfix releases that fix some bugs, I can revert back to 1.20.0. I did set 1.20.3 as it's the version you currently get when you run flutter channel stable, and you can check the bundled Dart SDK version by running dart --version or flutter --version. Latest stable Dart SDK is 2.9.3, but that one didn't make it to Flutter: flutter/engine#20838

[renefloor]

Yes if you can revert it that would be good. I don't want people to force a Flutter update when it is not needed for that package. It is mainly 1.20 because it is really incompatible with 1.17.

[Zazo032]

Re-opened in #464 with some analyzer changes that may help #463