Upgraded to Gatsby v3

This is a Gatsby v3 compatible version of the lumen starter, started by the folks at Gatsby Central.

Here's a little recommended reading for Gatsby v3.

• First, what's new in v3
• The Gatsby v2 announcement blog post

If you do decide to upgrade, these articles will help you do that.

• Updating Your Dependencies
• Handling Breaking Changes
• Future Breaking Changes
• For Plugin Maintainers
• Known Issues

Enjoy!