Add const constructor for Version

[letsar]

The Version class could have a const constructor. Then we could use this const construtor lines 47 and 67 of version.dart file.

[Solido]

Later we should be able to access multiple commits so a lot of class will be refactored and did not want to over optimise too early. This was wrote in a hurry last night so you can break stuffs if you want to test ideas. I would add const at the last time.