Maybe this is fixed already in github but sadly I'm limited to what is available on CRAN.
> library(httr)
> demo("gist")
demo(gist)
---- ~~~~
Type <Return> to start :
> gists_r <- GET("http://gist.github.com/api/v1/json/gists/hadley")
> gists <- content(gists_r)$gists
Loading required package: XML
> str(gists)
NULL
> gist_r <- GET(str_c("http://gist.github.com/api/v1/json/", gists[[1]]$repo))
Error in xor(is.null(url), is.null(handle)) :
could not find function "str_c"
Seems like this could be fixed if the demo loaded stringr first (str_c is there).