Use `fs.readFile` instead of `fs.readFileSync`

On [line 17 of `mergeYamlToJson/index.js`](https://github.com/d4nyll/mermal/blob/master/src/mergeYamlToJson/index.js#L17), use [`fs.readFile`](https://nodejs.org/api/fs.html#fs_fs_readfile_path_options_callback) instead of `fs.readFileSync`.