update to use new io.Seek consts, require min go1.7

tag.go

@@ -18,7 +18,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"os"
 )
 
 // ErrNoTagsFound is the error returned by ReadFrom when the metadata format
@@ -34,7 +33,7 @@ func ReadFrom(r io.ReadSeeker) (Metadata, error) {
 		return nil, err
 	}
 
-	_, err = r.Seek(-11, os.SEEK_CUR)
+	_, err = r.Seek(-11, io.SeekCurrent)
 	if err != nil {
 		return nil, fmt.Errorf("could not seek back to original position: %v", err)
 	}