--as-cran default results in warnings when pushing a released version

[wch]

If you release a version of a package to CRAN, and then push a commit where the version number is the same as that released version, you'll get a warning. For example, after ggvis 0.4.1 was accepted on CRAN, I merged the v0.4.1-rc branch into master. The result is a warning (or error when you use warnings_are_errors: true):
https://travis-ci.org/rstudio/ggvis/builds/56582281#L1194-L1196

I think that this shouldn't trigger an error, since the code is fine. Generally I think it's desirable to use the settings enabled by --as-cran, except for this version check.

In order to enable all the --as-cran settings without actually using --as-cran, one can set the environment variables listed at the bottom of this section of R Internals. That's what we do in devtools: https://github.com/hadley/devtools/blob/a07114c/R/check.r#L126-L159

[hadley]

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