Your error suggests that somehow ezDev either failed to install or
failed to load the stringr package. Try doing this yourself manually:
install.packages('stringr')
library(stringr)
library(RCurl); eval(expr =
parse(text=getURL('https://raw.github.com/mike-lawrence/ez/master/R/ezDev.R',ssl.verifypeer=FALSE)));
ezDev()