Travis: Install a matching version of Yarn
Travis: Install a matching version of Yarn

Since Heroku uses the version in `package.json` whereas until now
Travis would use an older version installed in their image.

KWierso/treeherder,KWierso/treeherder,edmorley/treeherder,edmorley/treeherder,KWierso/treeherder,edmorley/treeherder,edmorley/treeherder,KWierso/treeherder