Use `gem` as PURL type for Rubygems

[keshav-space]

Going by the PURL spec we should use gem as PURL type in fetchcode.package
https://github.com/nexB/fetchcode/blob/d0a3fa9bb56dc3a77f7d3d7bd5b8d0e40c7a8612/src/fetchcode/package.py#L307

More on this here aboutcode-org/purldb#247 (comment) by @johnmhoran