Upgrade to latest go-git version. (#124)

Do small changes to be able to use the last go-git v4 version.

Author: ajnavarro

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 sudo: false
 go:
- - 1.7
  - 1.8
+ - 1.9
  - tip
 matrix:
  fast_finish: true

commits.go

@@ -58,7 +58,7 @@ func (commitsTable) Children() []sql.Node {
 }
 
 type commitIter struct {
-i *object.CommitIter
+i object.CommitIter
 }
 
 func (i *commitIter) Next() (sql.Row, error) {

database_test.go

@@ -8,7 +8,7 @@ import (
 
 "github.com/src-d/go-git-fixtures"
 "github.com/stretchr/testify/assert"
-"gopkg.in/src-d/go-billy.v2/memfs"
+"gopkg.in/src-d/go-billy.v3/memfs"
 "gopkg.in/src-d/go-git.v4"
 "gopkg.in/src-d/go-git.v4/storage/filesystem"
 )