Ignore old build numbers in matrix builds

There's a bug in Jenkins which means old child builds may still be included in the XML returned for the current build, so for example `omego download --branch OMERO-5.0-merge-build ...` fails. This PR discards any builds which don't match the build number given in the root element's url tag.

Testing:
`omego download --branch OMERO-5.0-merge-build --skipunzip cpp -v` will fail without this PR, and should wokr on'ce its merged. Also test upgrade if you want.