Maintainer change

Hi,

I am nominating myself to continue maintenance on this project ;) It seems that no PRs were accepted since some time.

If you accept this PR, please also add me as owner to the gem at rubygems.org using this command, that we can keep the gem name:

    % gem owner -a rubygems@eggchamber.net rspec-xml

After this has been done I will release a new version of the gem including the currently open PRs. Please review my fork for details.

@benesch, @leomayleomay, @TVScoundrel: If one of you wants to maintain this, feel free to speak up ;)