Support preview releases with dev prefix like `1.0a1.dev2`

[pekkaklarck]

Currently rellu only supports final versions like 1.0, previews like 1.0.1a1 and dev versions like 1.0.1.dev1. This means that we need to go, for example, from 1.0a1 back to 1.0.dev1 when 1.0a2.dev1 would be better. This needs to be implemented!