Replace with already packaged version

--- a/plumbing/format/config/decoder.go
+++ b/plumbing/format/config/decoder.go
@@ -3,7 +3,7 @@
 import (
 	"io"
 
-	"github.com/go-git/gcfg"
+	"github.com/src-d/gcfg"
 )
 
 // A Decoder reads and decodes config files from an input stream.