It may be some time before the Ubuntu R packages are available on CRAN, from what I can tell by Googling a bit:
https://stat.ethz.ch/pipermail/r-sig-debian/2018-April/002829.html
https://twitter.com/search?q=%40marutterstat&src=typd
For the time being, you would need to downgrade to R 3.4.4 or lower in order to deploy to shinyapps.io