cmd/servegoissues: Update issues.Service interface.

[dmitshur]

Update for shurcooL/issues@d5974e9.

This change is needed to fix what otherwise would be a build error:

$ go build -o /dev/null github.com/bradfitz/go-issue-mirror/cmd/servegoissues
# github.com/bradfitz/go-issue-mirror/cmd/servegoissues
cmd/servegoissues/servegoissues.go:36: cannot use issuesService literal (type issuesService) as type "github.com/shurcooL/issues".Service in argument to issuesapp.New:
	issuesService does not implement "github.com/shurcooL/issues".Service (wrong type for ListComments method)
		have ListComments(context.Context, "github.com/shurcooL/issues".RepoSpec, uint64, interface {}) ([]"github.com/shurcooL/issues".Comment, error)
		want ListComments(context.Context, "github.com/shurcooL/issues".RepoSpec, uint64, *"github.com/shurcooL/issues".ListOptions) ([]"github.com/shurcooL/issues".Comment, error)