[Rd] R CMD build then check fails on R-devel due to serialization version

[Neal Richardson]

Hi,
Since yesterday I'm seeing `R CMD check --as-cran` failures on the
R-devel daily build (specifically, R Under development (unstable)
(2018-01-09 r74100)) for multiple packages:

* checking serialized R objects in the sources ... WARNING
Found file(s) with version 3 serialization:
‘build/vignette.rds’
Such files are only readable in R >= 3.5.0.
Recreate them with R < 3.5.0 or save(version = 2) or saveRDS(version =
2) as appropriate

As far as I can tell, revision 74099
(https://github.com/wch/r-source/commit/d9530001046a582ff6a43ca834d6c3586abd0a97),
which changes the default serialization format to 3, clashes with
revision 73973 (https://github.com/wch/r-source/commit/885764eb74f2211a547b13727f2ecc5470c3dd00),
which checks that serialized R objects are _not_ version 3. It seems
that with the current development version of R, if you `R CMD build`
and then run `R CMD check --as-cran` on the built package, it will
fail.

Neal