Actually use the header-link-hover-color variable.

I see a `$header-link-hover-color` defined [here](https://github.com/GumbyFramework/Gumby/blob/d4093cb734b54ef649cdd06fb78e6e53d8c20b55/sass/var/_settings.scss#L71), however it doesn't work as I would expect. In fact, I don't see it used anywhere else. I suspect it went unnoticed because by default it is the same value as `$body-link-hover-color`.

I have simply added what I expect was it's intended usage (changing the color of header links while hovering).