chore(deps): bump gatsby-source-graphql version

[solon]

When using this package in a project that uses a recent version of graphql (i.e. >= 14.0.0), there will be warnings about incorrect peer dependencies.

> warning "gatsby-source-graphql-universal > gatsby-source-graphql > [email protected]" has incorrect peer dependency "graphql@^0.11.3 || ^0.12.3 || ^0.13.0".

> warning "gatsby-source-graphql-universal > gatsby-source-graphql > [email protected]" has incorrect peer dependency "graphql@^0.13.0".

Version 2.1.15 of gatsby-source-graphql specifies more recent versions of apollo-link and graphql-tools which have updated peer dependencies that include the latest versions of graphql.

This PR bumps the gatsby-source-graphql dependency accordingly.