adding ruby gemspec support.

Signed-off-by: Toure Dunnon <toure.dunnon@anchore.com>

syft/pkg/gem_metadata.go

@@ -0,0 +1,7 @@
+package pkg
+
+type GemMetadata struct {
+	Name    string `mapstructure:"name" json:"name"`
+	Version string `mapstructure:"version" json:"version"`
+	// TODO: add more fields from the gemspec
+}