`repos` should contain named strings

Using `rocker/hadleyverse` on windows 7, I encountered the exact issue described [here](https://support.rstudio.com/hc/communities/public/questions/200871876-Problems-with-RStudio-v0-98b-Preview-Release): I can't use the RStudio buttons to install/update packages. This PR applies the fix from that RStudio support thread, which does indeed fix the problem for me.