title: Having issue with Cakephp Database Migrations
body: <p>I am trying to create a table using database migrations, here is my code</p>
  
  <pre><code>    public function change()
      {
          $table = $this-&gt;table('tags');
          $table-&gt;addColumn('title', 'string', ['default' =&gt; null,'limit' =&gt; 255,'null' =&gt; false,]);
          $table-&gt;addColumn('created', 'datetime', ['default' =&gt; null,'null' =&gt; false,]);
          $table-&gt;addColumn('modified', 'datetime', ['default' =&gt; null,'null' =&gt; false,]);
          $table-&gt;addUniqueKey('title');         // Giving error **Fatal error: Call to undefined method Phinx\Db\Table::addUniqueKey()**
          $table-&gt;create();
      }
  </code></pre>
  
  <p>I want to set <code>title</code> column unique, but when I try to do it then it gives the error:</p>
  
  <blockquote>
    <p>Fatal error: Call to undefined method Phinx\Db\Table::addUniqueKey()</p>
  </blockquote>