Migrate to Gatsby v2

[tw0517tw]

https://next.gatsbyjs.org/blog/2018-09-14-gatsby-v2/
https://next.gatsbyjs.org/docs/migrating-from-v1-to-v2/

[chentsulin]

https://github.com/gatsbyjs/gatsby/blob/master/docs/docs/migrating-from-v1-to-v2.md