Update for Gatsby 3

[davwheat]

This PR updates this plugin for Gatsby 3.

For now, run:

npm remove gatsby-plugin-react-head
npm i https://github.com/davwheat/gatsby-plugin-react-head

or

yarn remove gatsby-plugin-react-head
yarn add https://github.com/davwheat/gatsby-plugin-react-head

[davwheat]

@maciek-bejamas Is there any chance this could be merged in the future? If not, I'll publish my fork to npm so I don't need to use the reference to the git repo.