Merge pull request #4241 from heartinpiece/master

Check if Zsh is installed prior to installing Oh My Zsh

tools/install.sh

 set -e
 
+CHECK_ZSH_INSTALLED=$(grep /zsh$ /etc/shells | wc -l)
+if [ ! $CHECK_ZSH_INSTALLED -ge 1 ]; then
+  echo "\033[0;33m Zsh is not installed!\033[0m Please install zsh first!"
+  exit
+fi
+unset CHECK_ZSH_INSTALLED
+
 if [ ! -n "$ZSH" ]; then
   ZSH=~/.oh-my-zsh
 fi