`Veryl.toml` without `version` field

[taichi-ishitani]

I put Veryl.toml file to each simulation directories to vary generic args given to the DUT.
Example: https://github.com/taichi-ishitani/rice/blob/master/sim/core/sim_binary/Veryl.toml
For this usage, the version info is unnecessary.

However, the version field is a mandatly feild so the MetadataError error is reported if Veryl.toml does not include this field.

To suppor the above usage:

• Change the version field to an optional field
• Check existence of this field only when veryl publish is executed