Disable certificate check on github downloads (ctd.)

(2nd try as I forgot to create an extra branch for the pull request)

On systems where ca certificates are not up-to-date, perlbrew fails when trying to download files from github. I work-around is implemented here - all downloads have certificate checks disabled.
I know this is not a great solution, but it might be the only way for folks to get perlbrew running...
Should fix https://rt.cpan.org/Ticket/Display.html?id=92379 and https://rt.cpan.org/Ticket/Display.html?id=92380