Follow github changes after updating

`go vet` fails due to github library changes::

k8s.io/test-infra/ghclient
 ghclient/core.go:60:14: cannot use client.Repositories
   (type *github.RepositoriesService) as type repositoryService
   in field value:
 *github.RepositoriesService does not implement repositoryService
   (wrong type for ListCollaborators method)
   have ListCollaborators(context.Context, string, string, *github.ListCollaboratorsOptions)
      ([]*github.User *github.Response, error)
   want ListCollaborators(context.Context, string, string, *github.ListOptions)
      ([]*github.User, *github.Response error)

This patch makes ghclient follow these changes.

ghclient/wrappers.go

@@ -43,7 +43,7 @@ type pullRequestService interface {
 type repositoryService interface {
 	CreateStatus(ctx context.Context, org, repo, ref string, status *github.RepoStatus) (*github.RepoStatus, *github.Response, error)
 	GetCombinedStatus(ctx context.Context, org, repo, ref string, opt *github.ListOptions) (*github.CombinedStatus, *github.Response, error)
-	ListCollaborators(ctx context.Context, owner, repo string, opt *github.ListOptions) ([]*github.User, *github.Response, error)
+	ListCollaborators(ctx context.Context, owner, repo string, opt *github.ListCollaboratorsOptions) ([]*github.User, *github.Response, error)
 }
 
 type usersService interface {
@@ -144,11 +144,12 @@ func (c *Client) ForEachPR(owner, repo string, opts *github.PullRequestListOptio
 // GetCollaborators returns all github users who are members or outside collaborators of the repo.
 func (c *Client) GetCollaborators(org, repo string) ([]*github.User, error) {
 	opts := &github.ListOptions{}
+	copts := &github.ListCollaboratorsOptions{}
 	collaborators, err := c.depaginate(
 		fmt.Sprintf("getting collaborators for '%s/%s'", org, repo),
 		opts,
 		func() ([]interface{}, *github.Response, error) {
-			page, resp, err := c.repoService.ListCollaborators(context.Background(), org, repo, opts)
+			page, resp, err := c.repoService.ListCollaborators(context.Background(), org, repo, copts)
 
 			var interfaceList []interface{}
 			if err == nil {