Turns out to be a bit more complicated, because the library is obsolete, apparently, and probably installed in a non-standard location. But the bigger problem is that RStudio shouldn't require it, so the deb installation may have fetched you an out-of-date version.
Go directly to RStudio.com to download the deb file. I'm on a slightly different flavor of Ubunto and an HTML-initiated download prompts for an installer. Circle back if you have problems.