Add --rpm-verbatim-gem-dependencies flag

This should help folks who want the old-style gem-rpm conversions
* with this flag, old behavior: rubygem-name
* without flag, default behavior: rubygem(name)

The 'default' behavior has been this behavior since 0.4.23 for most
users.
If you changed the package name prefix via --gem-package-prefix or
--gem-package-name-prefix then this behavior was broken.
It was fixed in #585.

But, I don't want folks relying on old behavior to be screwed
when that behavior mysteriously vanishes! This should be a good
enough work around for now.

Reported by Ryan Doherty via the fpm-users mailing list.