Upgrade Mapbox Studio to v0.1.5

The latest version addresses bugs
related to atom-shell integration.

For a full list of changes, see
https://github.com/mapbox/mapbox-studio/blob/mb-pages/CHANGELOG.md#changelog
Merge pull request #6811 from tristen/mbstudio-v0.1.5

Upgrade Mapbox Studio to v0.1.5