Fix win-builder R devel errors

[andybega]

R CRAN check fails on win-builder with R devel, see https://gist.github.com/andybega/abed6a8dc097f86d936f61b377480e63:

library("ggplot2")

df <- structure(list(date = structure(c(17532, 17897, 18262, 18993, 
19358, 19723, 20089), class = "Date"), id = c("9991", "9991", 
"9991", "9992", "9992", "9992", "9992"), cowcode = c(999, 999, 
999, 999, 999, 999, 999), y = c(1, 1, 1, 2, 2, 2, 2)), row.names = c(NA, 
-7L), class = "data.frame")

ggplot(df, aes(x = date, y = y, group = cowcode)) + geom_line()

# Relevant portion of the trace:
# 5: ggplot2::scale_y_discrete(breaks = brks)
# 6: discrete_scale(c("y", "ymin", "ymax", "yend"), "position_d", identity, ..., expand = expand, 
#        guide = "none")
# 7: structure(list(call = match.call(), aesthetics = aesthetics, scale_name = scale_name, 
#         palette = palette, range = DiscreteRange$new(), limits = limits, na.value = na.value, 
#         expand = expand, name = name, breaks = breaks, labels = labels, legend = legend, 
#         drop = drop, guide = guide), class = c(scale_name, "discrete", "scale"))
# 8: DiscreteRange$new()
# 9: stop("`inherit` must be a R6ClassGenerator.")
#
# ...
#
# Message: `inherit` must be a R6ClassGenerator.

[andybega]

Here are related SO questions:

https://stackoverflow.com/questions/54138329/ggplot2-claims-that-inherit-must-be-a-r6classgenerator

https://stackoverflow.com/questions/51675464/what-is-discreterangegenerator-error-in-ggplot