auto-remove README.pod, if present

...as we're now using the markdown version.
dzil: drop ExtraTests, add RunExtraTests and CheckExtraTests

I think I'd prefer the author/release tests to stay under xt/; these should
allow both that and for them to still be run as appropriate.
travis: --quiet cpanm
drop ExtraTests from the prereqs listing
move the unincorporated prereqs over into dist.ini

...as we really don't need to load them each time, but would like to have them
around.
dzil: add SchwartzRatio