add the param version to write_rds()

[shrektan]

Since serialization format 3 is the default on R 3.6.0, our users may want specify the rds serialization format they are using, due to the fact that format 3 can't be read by R version before 3.5.0.

[jennybc]

For context, when faced with a related situation in usethis::use_data(), I chose to expose the version but default to 2. It's worth considering whether readr should do same.

https://usethis.r-lib.org/reference/use_data.html

r-lib/usethis@12392c5

[jimhester]

I think that we should default to 2 as well here, as it has greater compatibility.

Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

Also could you re-run the roxygen documentation and commit the results, either devtools::document() or roxygen2::roxygenize().

Otherwise this looks good, thanks!

[shrektan]

@jimhester I've changed the default value of version to 2, run the docs and added an item in NEWS. Thanks. :D

[shrektan]

BTW, the failed checks are caused by other reasons and not related to this PR.

[jimhester]

A great improvement!