More compatible way of checking if zsh is installed

tools/install.sh

@@ -24,12 +24,10 @@ main() {
   # which may fail on systems lacking tput or terminfo
   set -e
 
-  CHECK_ZSH_INSTALLED=$(grep /zsh$ /etc/shells | wc -l)
-  if [ ! $CHECK_ZSH_INSTALLED -ge 1 ]; then
+  if [ -z `which zsh` ]; then
     printf "${YELLOW}Zsh is not installed!${NORMAL} Please install zsh first!\n"
     exit
   fi
-  unset CHECK_ZSH_INSTALLED
 
   if [ ! -n "$ZSH" ]; then
     ZSH=~/.oh-my-zsh