Replace :erb filter with plain HAML.

Rendered HTML is the same.

We use HAML exactly avoid ERB or pure HTML, so let's not use `:erb`.

There is another occurence which I cannot get rid of at: https://github.com/gitlabhq/gitlabhq/blob/c2c41fb2d3b6b5934ff26cec77c15f6c45351018/app/views/projects/blame/show.html.haml#L31 because of https://github.com/haml/haml/issues/648