Get version from manifest

The version is written to the JAR manifest during packaging. We can get is
from there with `Package.getImplementationVersion()`, so we do not need to adjust the hardcoded `Version.VERSION` string manually before making a release.

Obviously, this will only work if the classes are loaded from the JAR. Otherwise (e.g. within IDE), the version will be undefined (`UNKNOWN-SNAPSHOT`), which I think is fine.