Some po mcleanup

Hi Bela,
I've hardcoded the Maven plugin versions, that's handy to make sure the builds are reproducible, both across different computer and across time.

Also updated some test dependencies, as _master_ wouldn't compile. Now it compiles but it still fails some tests so maybe these dependency updates should be considered with care. Feel free to cherry-pick only the commits you feel more confident about, I've kept each change as cleanly separated as possible.