Merge pull request #2827 from ncanceill/easymerge

...you folks are amazing. :-)

"Easy-to-merge" 

tools/check_for_upgrade.sh

@@ -5,7 +5,7 @@ function _current_epoch() {
 }
 
 function _update_zsh_update() {
-  echo "LAST_EPOCH=$(_current_epoch)" > ~/.zsh-update
+  echo "LAST_EPOCH=$(_current_epoch)" >! ~/.zsh-update
 }
 
 function _upgrade_zsh() {
@@ -22,6 +22,10 @@ fi
 
 [ -f ~/.profile ] && source ~/.profile
 
+# Cancel upgrade if the current user doesn't have write permissions for the
+# oh-my-zsh directory.
+[[ -w "$ZSH" ]] || return 0
+
 if [ -f ~/.zsh-update ]
 then
   . ~/.zsh-update