Nested layouts `wrap_layout` needs to be outputted

In order for `wrap_layout` to properly work, it needs to be outputted to
using `<%=`. It seems that this is something that recently came about
as it previously was not required. I found this while trying to upgrade
a middleman 3.1 site to 3.3.
Merge pull request #353 from dgalarza/dg-update-nested-layouts

Nested layouts `wrap_layout` needs to be outputted