CI: install yarn like TerriaJS

[pjonsson]

This copies the more recent CI-setup from TerriaJS, which eliminates the need to keep a copy of an old version of yarn in the repository.

This eliminates a lot of warnings when doing a project scan for problems in WebStorm.

Edit: problems detected by WebStorm with default settings before/after: 629 errors, 4819 warnings, 14459 weak warnings vs 628 errors, 593 warnings, 1018 weak warnings. The scan time also goes from taking minutes on my desktop machine to passing in a few seconds.

[pjonsson]

This repository never seems to run the CI until one of you approve it. Can the CI settings be changed so I can make the PR green before you have to look at it?