add check before just installing brew-cask

.profile

@@ -94,7 +94,7 @@ F="$HOME/liquidprompt/liquidprompt";  ! [ "$PS1" -a -f "$F" ]  ||  . "$F"
 _install_homebrew() {
 	xcode-select --install 2>&1 | grep -q "already installed"  ||  exit
 	[ -d "$HOME/homebrew" ]  ||  (mkdir "$HOME/homebrew" 2>/dev/null  &&  curl -L "https://github.com/Homebrew/homebrew/tarball/master" | tar xz --strip 1 -C "$HOME/homebrew"  &&  brew update)
-	brew install caskroom/cask/brew-cask
+	which brew-cask >/dev/null  ||  brew install caskroom/cask/brew-cask
 }
 
 _install_tools() {