supporting the github version of ggplot2

[xiangpin]

Thanks for the great package. The github version of ggplot2 has some significant changes. But the current version of gghalves is not compatible with it. The request has solved the problem.

> library(gghalves)
Loading required package: ggplot2
> example(geom_half_boxplot)

gm_hl_> ggplot(iris, aes(x = Species, y = Petal.Width, fill = Species)) +
gm_hl_    geom_half_boxplot()
Hit <Return> to see next plot:

gm_hl_> ggplot(iris, aes(x = Species, y = Petal.Width, fill = Species)) +
gm_hl_    geom_half_boxplot(side = "r")
Hit <Return> to see next plot: