fish: ensure ruby is installed for greeting

fish/prompt

@@ -19,7 +19,7 @@ function fish_right_prompt
 end
 
 function fish_greeting
-	if test -f /usr/share/dict/words && test (which shuf)
+	if test -f /usr/share/dict/words && test (which shuf) && test (which ruby)
 		set_color c50ed2
 
 		echo -n "Word of the moment: "(shuf -n1 /usr/share/dict/words) |