Commit order is neither respected by travis nor by the actual builder. If all the CRAN packages _directly_depend on R at least the builder should dtrt.
The issue for this failure is the dependency chain: R-cran-scales -> R-cran-munsell -> R-cran-colorspaces.
xbps-src only tracks dependencies only one level deep. So R-cran-colorspaces isn't necessarily built before R-cran-scales. The usual "fix" is to push the updates in the correct order, or as a workaround make R-cran-scales directly depend on R-cran-colorspaces.