Dart support

This will add support for [Dart](https://www.dartlang.org/).

```yaml
language: dart
```

You can provide the version to use with the `dart` property. `stable` is the default and you can use either `dev` or a version number.

If the project contains a `pubspec.yaml` file then `pub get` is launch.

Finally the script run `dart test/all_test.dart` if `test/all_test.dart` exists.
Sorry, but I changed the labels to have prefix `community:`; e.g., https://github.com/travis-ci/travis-ci/issues/new?labels=community:julia