Bake version number into source.

[mtbc]

The current version check relies on the Gateway being packaged with its original MANIFEST.MF which, being intended for use as a library, it may no longer be packaged with, depending on what JAR reassembly, package renaming, uberjar approach, etc. is done by downstream build processes. Given the criticality of the version check, a more robust approach may be warranted. For example, one could apply a Gradle equivalent of ome/omero-downloader#44, perhaps reading a filtered properties file from omero-blitz.