Test for attachment requirement
Prevent publishing of publications without attachments

Allows either HTML or normal attachments.
I didn't extended this up to Publicationesque as that doesn't include HTML attachments.

https://www.pivotaltracker.com/story/show/49121005
Add html attachment to test publication factory

Without this the new validation prevents the factory creating published
publications.
The build strategy should avoid an extra write to the database that
would slow our tests down.
Fix tests for optional html attachments.

This has been broken as a side effect of choosing to make the
publication factory use html attachments as a simple way of making the
new validation rules pass by default. This patch reverses the effect of
that change for these specific tests that deal with a lack of html
attachment.
Fix test for new html version slug

Requirement for an attachment for published versions broke this test.
This has been fixed by starting with a normal attachment and then
swapping it for an html attachment.