main: Destroy GjsContext before exit

Here are a few places where I missed it before: the GjsContext must be
unrefed before exiting, or it will crash on exit.

https://bugzilla.gnome.org/show_bug.cgi?id=778425

src/main.c

@@ -360,6 +360,7 @@ list_modes (const char  *option_name,
   if (!gjs_context_eval (context, script, -1, "<main>", &status, NULL))
       g_message ("Retrieving list of available modes failed.");
 
+  g_object_unref (context);
   exit (status);
 }