Update recommendations for installing Vagrant on Ubuntu
Update recommendations for installing Vagrant on Ubuntu

This is a rebased version of #1099 by @chadmiller, which was
unfortunately bitrotted during the transition to Read the Docs.

Closes #1099.

garrettr/securedrop,heartsucker/securedrop,micahflee/securedrop,ageis/securedrop,conorsch/securedrop,pwplus/securedrop,ehartsuyker/securedrop,pwplus/securedrop,ehartsuyker/securedrop,ageis/securedrop,pwplus/securedrop,micahflee/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ageis/securedrop,pwplus/securedrop,micahflee/securedrop,ageis/securedrop,pwplus/securedrop,conorsch/securedrop,garrettr/securedrop,garrettr/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,conorsch/securedrop,conorsch/securedrop,pwplus/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop