Add homepage field as output to the gemspec metadata (#215)

https://github.com/anchore/syft/issues/214

Signed-off-by: Toure <tdunnon@gmail.com>

syft/pkg/gem_metadata.go

@@ -6,4 +6,5 @@ type GemMetadata struct {
 	Files    []string `mapstructure:"files" json:"files"`
 	Authors  []string `mapstructure:"authors" json:"authors"`
 	Licenses []string `mapstructure:"licenses" json:"licenses"`
+	Homepage string   `mapstructure:"homepage" json:"homepage"`
 }