fix: audit_r_versions() returns a bar chart instead of a histogram

[csgillespie]

r_versions are a factor not a numeric. So we should use a bar chart. The actual graphs look the same, but using a barchart removes the warning

Warning message:
In ggplot2::geom_histogram(ggplot2::aes(r_version), stat = "count") :
  Ignoring unknown parameters: `binwidth`, `bins`, and `pad`