pronto-eslint depends on eslintrb, which expects the `.eslintrc` file
in JSON format. So we're migrating the `.eslintrc.yml` file to JSON and
introducing a symbolic link so the `.eslintrc` file points to
`.eslintrc.json`.
We could also use pronto-eslint_npm, but unfortunately it's not
maintained anymore and the latest version is not compatible with Pronto
0.11.
