Version is not always derived from a tag in Travis CI

[2m]

As a default, Travis CI fetches only 50 last commits when it checks out repository before the build.

If the last tag is more than 50 commits old then sbt-dynver falls back to having only the short commit sha as the version name, like 50-aee9ebeb. This is problematic when releasing snapshots from Travis.

Also previousStableVersion becomes None. This, with combination with MiMa, might end up in a situation where no BC checks are being run.

Currently the workaround is to fetch full history in the before_install section of Travis configuration:

before_install:
  - git fetch --unshallow

It would be great to have a setting in sbt-dynver which would prevent versions being derived if no tags are present.