make `previousArtifact` more robust

When `sbt-release` tags a release, it doesn't write our custom key
`scalazMimaBasis` to `version.sbt`. For version 7.0.4 (fb6252e), this led to
SBT giving an error message about an undefined key.

Solution: Make `scalazMimaBasis` optional.
Setting version to 7.0.5
Setting version to 7.0.6-SNAPSHOT