If you're deploying from Travis CI to shinyapps.io (yes, that's a thing :), remember that you can always hotfix this problem in your .travis.yml:
language: r
r:
- 3.4.4
# this makes travis-ci (which runs on ubuntu) use the last available R version.
Full R travis docs here.
Just remember to delete the hard-coded R version once this problem is solved 
.
Thanks as always to @josh and the entire RStudio team for the amazing work. 