Add Gatsby peer dependency version

[loreina]

Running into this npm warning when running a command:

npm WARN gatsby-source-itchio@0.1.1 requires a peer of gatsby@ but none is installed. You must install peer dependencies yourself.

I think this could be solved by adding a version to the Gatsby peer dependency of the plugin. Glanced at the package.json and the version is blank. I'm using gatsby@^2.20.3 and it's working great.