use_tidy_test_env()

[hadley]

Would create block to copy at top of cran-comments.md:

## Test environments
* local OS X install, R 3.5
* travis-ci: R 3.1, 3.2, 3.3, 3.4, release, devel.
* win-builder: R devel

[hadley]

And perhaps use_tidy_ci() could print it out too.