Format of versions of antikythera instances and gears.

The format rule is a stricter variant of semantic versioning; pre-release part and build metadata part are filled with information from the current git commit (committer date and commit hash, respectively). This way we always make a new version for each new git commit, which enables both antikythera instances and gears to be deployed on a per-commit basis. The actual version strings are created by Antikythera.MixCommon.version_with_last_commit_info/1 defined in mix_common.exs file.

Note that the current format prohibits multi-digits numbers as major/minor/patch version; this is just to simplify deployment and not an intrinsic limitation.