Packer push fails due to pushed box version not following SemVer

[TuningYourCode]

Hi,

we had the issue that we pushed a box to phagrancy as "20201224" (or something similar ;)).

That's a totally valid version as of https://github.com/TuningYourCode/phagrancy/blob/5aef5376d114f0d021f7b690c8e18ef3050211c7/src/Model/Input/ValidatesVersion.php#L31

Sadly the following push by packer gave us an error because of something like "versions.version of type number is not of type string".
We thought that we pushed a new false version but packer parses all existing versions before it will push new boxes.

As per https://www.vagrantup.com/docs/boxes/versioning#version-constraints versions have to be strictly in format "X.Y.Z" - even if they don't enforce it on their api themself (https://app.vagrantup.com/TuningYourCode/boxes/debian-buster64-puppet6).

That would be a breaking change in the api but would probably prevent new starters from doing the same mistake and headache during debugging.

@dlundgren if you agree i could prepare a PR to sharpen the version validation.

Best regards,
TuningYourCode

[dlundgren]

A PR sounds great, sorry for the delay