Fix the build with the most recent go-git

cmd/hercules/main.go

@@ -45,7 +45,7 @@ import (
 	"github.com/vbauerster/mpb"
 	"github.com/vbauerster/mpb/decor"
 	"golang.org/x/crypto/ssh/terminal"
-	"gopkg.in/src-d/go-billy.v3/osfs"
+	"gopkg.in/src-d/go-billy.v4/osfs"
 	"gopkg.in/src-d/go-git.v4"
 	"gopkg.in/src-d/go-git.v4/plumbing/object"
 	"gopkg.in/src-d/go-git.v4/storage"