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.
I'd imagine the default is to install whatever the latest stable SDK is.

However, I'm guessing most developers will want to lock to a specific version of an SDK. For example, "I want to run all my CI and tests with version X.Y.Z of the SDK". I guess I specify that in my travis.yaml file?