Make text black in error/danger banners

For consistency with what GOV.UK Elements does, the text in our banners
should be black, not red. See examples here:
http://govuk-elements.herokuapp.com/errors/#summarise-errors

This also makes us consistent with what Pay are doing.

app/assets/stylesheets/components/banner.scss

@@ -49,7 +49,7 @@
   @extend %banner;
   @include bold-19;
   background: $white;
-  color: $error-colour;
+  color: $text-colour;
   border: 5px solid $error-colour;
   margin: 15px 0;
   text-align: left;