Add 'old' Tag for Last Version

[surmann]

It would be nice to have a tag for the last R version.

I test my package against 'devel', 'latest', and '3.4'. Progression to R 3.6 we have to update all our yml files from tag '3.4' to '3.5'. Here it would help to have the new tag 'old', for instance.

[nuest]

In line with CRAN, this tag could be oldrel, and one can probably add this to the post_push tagging mechanism.

[cboettig]

Thanks, keep meaning to get back to this but haven't done it. agree we just need a post_push hook to do this. @nuest want to send a PR for this?

[surmann]

Is it easy to write a peace of code which rolls the oldrel version forward related to the current version of R?

[cboettig]

@surmann Yes, I think we just need to add the relevant logic to the post_push hook to create and push the extra tag. See https://github.com/rocker-org/rocker-versioned/blob/master/r-ver/hooks/post_push and the Docker docs on hooks. -- PRs welcome. Ideally this could be done in a way that doesn't require the script to be updated manually on each new R release...