Parse version of dart2js compiled sass compiler

[xanderio]

This allows users of platforms where the dart SDK is not available (e.g. FreeBSD) to use the dart2js version which can be installed via npm install -g sass.

Checklist

• [x] Updated CHANGELOG.md describing pertinent changes.
• [ ] Updated README.md with pertinent info (may not always apply).
• [ ] Updated site content with pertinent info (may not always apply).
• [x] Squash down commits to one or two logical commits which clearly describe the work you've done. If you don't, then Dodd will 🤓.