Add new Cloud tag so that we can build the recent Heroku docs

Switch preview to chunked transfer (elastic#921)

Switches the PR preview infrastructure from buffering the entire file to
chunked transfer. This allows us to do two things:
1. Shrink the size of the node process significantly.
2. Handle large files.

Integtest: Add tests for building all books (elastic#810)

Adds a basic test for building all books. I expect we'll want to refactor
it significantly as we port more tests for Make to rspec, but this is a
start!

Revert .NET API's branch to 6.x (elastic#582)

The .NET API doesn't branch the same way as the stack. It looks like
most of the clients don't actually. We bumped its branch to 6.7 as part
of bumping the stack. This was incorrect because they have no plans to
create a 6.7 branch.