Install DBIC dev rel under CLEANTEST="false"

The stable one has a broken Storage::DBI::ODBC::Firebird (missing
true value).
Test with more perl versions
Allow 5.8.3 runs to fail

Several upstream deps are flaky, and nobody has the tuits to fix them.
Install Math::BigInt, 5.8.x ship a too old one
Add --verify to PERL_CPANM_OPT to catch corrupt tarballs

<@ribasushi> ilmari: experiment with mirrors
<@ribasushi> the metacpan proxy of the download part (why?!) is
             sometimes giving back shortened tarballs, which happen to
             extract fine
<@ribasushi> but the source is in fact fucked