Gets default version configuration of this rule.

This finds and reads 'package.json' file, then parses 'engines.node' field.
If it's nothing, this returns null.

@param {string} filename - The file name of the current linting file.
@returns {string} The default version configuration.
function getDefaultVersion(filename) {
    const info = getPackageJson(filename)
    const nodeVersion = info && info.engines && info.engines.node

    return semver.validRange(nodeVersion) || DEFAULT_VERSION
}