warn user that installation will fail if she doesn't have necessary write permission

Author: Mizou04

bin/ruby-install

@@ -18,6 +18,11 @@ fi
 
 init || exit $?
 
+if [[ -w "$install_dir" ]]; then
+	error "seems you don't have necessary write permission on \"$install_dir\""
+	exit 1
+fi
+
 if [[ $no_reinstall -eq 1 ]] && [[ -x "$install_dir/bin/ruby" ]]; then
 	log "Ruby is already installed into $install_dir"
 	exit