change default shell to zsh

because it's better

screen/screenrc

@@ -41,7 +41,7 @@ msgwait               15
 pow_detach_msg        "BYE"
 
 # Set the default program for new windows.
-shell                 bash
+shell                 zsh
 
 # Default timeout to trigger an inactivity notify.
 silencewait           30              # default: 30