fix issue that prevented update of zsh plugins

check_for_update.sh

@@ -15,6 +15,9 @@ function _update_dotfiles_update() {
 }
 
 function _upgrade_dotfiles() {
+    if [ -z $ZSH ]; then
+        ZSH="$HOME/.oh-my-zsh" # TODO: this is hard-coded, find a better way to do this
+    fi
     (cd $DOTFILES; git pull -q --rebase  && echo "Succesfully upgraded dotfiles" || echo "Could not upgrade dotfiles.")
      # amix's vimrc update
      amix_vimrc=$HOME/.vim_runtime