install USAboundariesData from GitHub

[dongzhuoer]

USAboundariesData is not available in CRAN (at least for R 3.6.1)

> install.packages("USAboundariesData")
Installing package into ‘/home/zhuoer/.local/lib/R’
(as ‘lib’ is unspecified)
Warning message:
package ‘USAboundariesData’ is not available (for R version 3.6.1) 

As djnavarro adds (76297b6) http://packages.ropensci.org to repos for Travis, the DESCRIPTION file also needs to change when remotes::install_github("hadley/ggplot2-book").

[hadley]

Thanks!