1 Trying this option worked for me.
 2 
 3 library(httr)
 4 with_config(use_proxy(...), install_github(...))
 5 
 6 OR
 7 
 8 library(httr)
 9 set_config(use_proxy(url = "******_", port = "_*"))
10 devtools::install_github("username/packagename")