[r] hotfix for release directory JSON parsing problem

[mlin]

Our way of converting the release directory JSON to a convenient data frame was too brittle for this week's updated release_directory.json (adding the do_not_delete attribute). Fix this by switching from rbind to the more flexible dplyr::bind_rows. (Our package already transitively depended on dplyr via Seurat and others.)

[mlin]

@pablo-gar @ebezzi FYI- this is somewhat urgent

[ebezzi]

Thanks Mike