(travis) Revisit the CI test matrix, and multiple improvements

- Add 5.8.4 testperl and also make the travis 5.8 run first: will
  give a much faster reponse time if something 5.8 related fails

- Make sure the perl sitelib sanitizer doesn't fire on legacy perls:
  on anything older than 5.12 we are essential nuking the baselib.
  The aftermath is that nobody will go to space that day.

- Stop considering DBIC_*_DEBUG part of the POISON-set
  These are more of a tracer - specify them explicitly instead