add flexboxgrid-helpers to head

Author: lgg

sources/html/layouts/head.jade

@@ -37,6 +37,7 @@ head
     <!-- Styles -->
     link(rel='stylesheet', href='assets/css/normalize.css')
     link(rel='stylesheet', href='assets/css/flexboxgrid.min.css')
+    link(rel='stylesheet', href='assets/css/flexboxgrid-helpers.min.css')
     link(rel='stylesheet', href='assets/css/style.min.css')
     <!-- Scripts -->
     //script(type='text/javascript' src='build/js/js.min.js')