Update vagrant_box_version to <= 201801.02.0

[tangrufus]

See: https://app.vagrantup.com/bento/boxes/ubuntu-16.04

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Question:
The box is ubuntu-16.04. Should we use lower-bound version constraint instead of upper-bound?
I.e.: Will this be better?

vagrant_box_version: '>= 201801.02.0'

[swalkinshaw]

>= makes it easier for people to keep their box up to date and should be fine, but I guess there's a small chance some update could break Trellis. I would assume the likelyhood of that happening would be very low though?

[swalkinshaw]

Going to merge this as is for now and we can see about flipping it after.