Bump min SDK to workaround broken analyzer (dart-lang/stream_transform#112)

There is a broken version of `package:analyzer` that causes issues on
travis for the `2.6.0` SDK. There is relatively little need to maintain
support for an SDK that old, so bump the minimum rather than pin
analyzer and be unable to pick up newer versions.

https://github.com/dart-lang/sdk/issues/42888

Fix formatting in changelog.