Respect rubygem(package) form with custom gem package name prefix

Hi Jordan,
I'm using fpm (great tool btw!) for packaging **GEMs -> RPMs** with Red Hat Software Collections. The problem is that fpm does not respect the standard format **rubygem(packagename)** for requires/provides, if any other prefix than standard "rubygem" is used for gems naming. This should fix it and keep the bracketed form even when different prefix is used for naming, like `--gem-package-name-prefix ruby193-rubygem` for Red Hat SCLs.

Cheers .st