Incompatibility with sf package

[ltalluto]

If I try to load viridis and the sf package in the same R session, I get an error on trying to load the second package. The error message is:

Error in get(genname, envir = envir) : object 'group_map' not found
Error in get(genname, envir = envir) : object 'group_split' not found

Whichever package is loaded first works fine. I am running sf ‘0.7.4’, viridis 0.5.1, and R 3.5.1 on macOS 10.14.5.