Fix missing use

I don't speak much English.

[![Build Status](https://travis-ci.org/gold1/cakephp.png?branch=3.0-model-missing-use)](https://travis-ci.org/gold1/cakephp)

### Problem
Fatal error: Class 'Cake\Model\ConnectionManager' not found in /lib/Cake/Model/Model.php on line 3291

### Solution
Add `use ConnectionManager` to Model.php.
`use Cake\Database\ConnectionManager;`

### Memo
'Cake\Model\ConnectionManager' moves to 'Cake\Database\ConnectionManager'

### Comment
Since I do not understand specification well, I have a possibility of being wrong.