Change dartfmt to dart format

[Larry Aasen]

The recommended Dart format command is now dart format instead of dartfmt. The file docs-travis-ci-com/user/languages/dart.md should be updated to reflect this.

[Koen Van Looveren]

Any idea on how we can get this to work?

This was our old config:

language: dart
dart:
  - stable
dart_task:
  - test
  - dartanalyzer: --fatal-warnings .
  - dartfmt

This is our new config:

language: dart
dart:
  - stable
dart_task:
  - test
  - dartanalyzer: --fatal-warnings .
  - format .

But the format command in the new config just runs dart test but I have no idea why?