Fix instructions involving Vagrant

[cceckman]

Going through the basics, I get:

∵ make docker-machine
vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'bento/ubuntu-16.04' could not be found. Attempting to find and install...
    default: Box Provider: virtualbox
    default: Box Version: >= 0
The box 'bento/ubuntu-16.04' could not be found or
could not be accessed in the remote catalog. If this is a private
box on HashiCorp's Atlas, please verify you're logged in via
`vagrant login`. Also, please double-check the name. The expanded
URL and error message are shown below:

URL: ["https://atlas.hashicorp.com/bento/ubuntu-16.04"]
Error: The requested URL returned error: 404 Not Found
Makefile:21: recipe for target 'vagrant' failed
make: *** [vagrant] Error 1
∴ 2

It looks like atlas.hashicorp.com is deprecated - it shows:

"The Packer, Artifact Registry and Terraform Enterprise (Legacy) features of Atlas will no longer be actively developed or maintained and will be fully decommissioned on Friday, March 30, 2018."

Is there a better source for images that isn't deprecated / enterprise / etc?

Is there an alternative to Vagrant?

(Mentioning here, but it appears to apply to the upstream as well.)

[DieterReuter]

Hmm, I'm easily build all the things now on Travis. I think it's a long time ago I used Vagrant - that's the method we started months ago.

[cceckman]

Is this just a docs issue, then - that this section is unnecessary?