Add maven plugin versions to silence maven warnings.

[norswap]

Fixes #26 & more.

[graalvmbot]

Hello Nicolas "Norswap" Laurent, thanks for contributing a PR to our project!

We use the Oracle Contributor Agreement to make the copyright of contributions clear. We don't have a record of you having signed this yet, based on your email address norswap -(at)- gmail -(dot)- com. You can sign it at that link.

If you think you've already signed it, please comment below and we'll check.

[norswap]

Hello Nicolas "Norswap" Laurent, thanks for contributing a PR to our project!

We use the Oracle Contributor Agreement to make the copyright of contributions clear. We don't have a record of you having signed this yet, based on your email address norswap -(at)- gmail -(dot)- com. You can sign it at that link.

If you think you've already signed it, please comment below and we'll check.

I'm an Oracle employee. The commit were done with my default git config (so no oracle email) but I can amend them to use that if it's more convenient.

[norswap]

Didn't notice there were pull requests open for this already, my bad.
cf. #83 (comment)