R: Add new env var to r.rb

[nealrichardson]

In September, an "experimental" environment variable was added to the development version of R to make package checks fail when the && or || operator was used inappropriately. CRAN appears to have enabled this in their incoming package checks, a fact I discovered when a submission to them was rejected yesterday. Setting this env var in my .travis.yml reproduced the failure on Travis, and we were able to fix and successfully resubmit the package to CRAN.

Since others may be affected by this policy change, I thought it would be good to add this environment variable to the standard R build configuration.

cc @jimhester @jeroen

[jimhester]

One issue is setting it to true is that if any of your packages dependencies trip the error they will fail and you won't be able to build your package.

Recent versions of R should turn this (only for the package being checked) with --as-cran which travis uses by default, so we may not have actually do anything.