Self-cleaning is not yet entirely stable - back it out for the time being

As a side-effect resolves RT#86267
Turns out Travis CPAN mirror is not local, add a fallback

See https://github.com/travis-ci/travis-cookbooks/commit/143a57d2ecee#diff-1

Sideffect of an empty PERL_CPANM_OPT was tickling this bug:
https://github.com/miyagawa/cpanminus/issues/288
Allow spaces in parallel installation module spec
Temporary inslatt Pod::POM via cpan

Pod::POM installation tickles a toolchain deficiency
https://rt.cpan.org/Ticket/Display.html?id=87124
https://github.com/miyagawa/cpanminus/issues/291
Fix test error failing after "undef\n=>" was fixed to stringify

http://perl5.git.perl.org/perl.git/commit/21791330af
Install EU::CB in a separate step as to not tickle an old CPAN bug

Older CPAN.pm had a problem with the circular dependency resolution firing
erroneously *after* everything was successfully installed, e.g.:

https://s3.amazonaws.com/archive.travis-ci.org/jobs/9336470/log.txt

(scroll to the end)
Ignore stderr in extract_prereqs - only rely on $?
perlbrew exits with 0 if `perlbrew use` fails :(