[gatsby v2] Change modifyBabelrc to onCreateBabelConfig

[Yuki Takemoto]

With the release of Gatsby v2, we will have to change modifyBabelrc to onCreateBabelConfig.

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

so I want to make pull request and publish alpha version for v2.

[Sébastien Lorber]

seems to be done by #4

[Yuki Takemoto]

Thanks!