Fix CRAN checks

[larmarange]

broom.helpers should pass all tests.

• a new version of gert package was just released but is not yet available for Windows and MacOS, cf. https://cran.r-project.org/package=gert => GitHub checks fail because all depencies cannot be installed
• there are warnings for Solaris and MacOS (cf https://cran.r-project.org/web/checks/check_results_broom.helpers.html ) It seems to be related with the rebuilding of the vignette
• R 4.0.4 not yet available for some platforms. Mays just a question of days

[larmarange]

For the vignette, it seems that in some contexts, recent version of pandoc is not available and therefore the vignette is build with Rmarkdown v1

Warning in engine$weave(file, quiet = quiet, encoding = enc) :
     Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.

paged_table() was introduced only in v1.2 and the problem appears when we call paged_table(). We could try to test the version number of rmakdown