- added second alternative: MavenVersionManager
- added test for MavenVersionManager

- renamed the Test class accordingly
- made semver4j an optional dependency
- updated javadoc of Semver4jVersionManager to make it clear that the semver4j library must be included

- added dependency semver4j
- added SemverJVersionManager to prevent breaking change for existing users (default implementation is still DefaultVersionManager => explicit opt-in required)
- added isStable to VersionManager interface making it easier to only allow snapshots in test environments(DefaultVersionManager returns always true)
- added test method for DefaultVersionManager for snapshots
- added tests for SemverJVersionManager

Thanks!