Support cataloging gemspec files

[alfredodeza]

What would you like to be added: Cataloging Ruby packages installed by gem - which requires parsing of gemspec files.

Why is this needed: To support parity with Anchore Engine that does parse gemspec, specifically related to this issue anchore/anchore-engine#629

Additional context: A gemspec parser will need to be put in place, similar to this:

// New returns a new Bundler cataloger object.
func New() *Cataloger {
	globParsers := map[string]common.ParserFn{
		"**/Gemfile.lock": parseGemfileLockEntries,
		".*specifications.*\.gemspec$": parseGemSpecEntries,
	}