Fixed compatibility with PHP 5.5

Hi, I found a bug in Codeception 2.0. When I put `Mockery::mock(\DateTime::class);` into my test I get this error:

```
Notice: Uninitialized string offset: 1 in ... \vendor\codeception\codeception\src\Codeception\SuiteManager.php on line 253
```

Note that the new PHP 5.5 `::class` feature is used. This commit is a quick workaround I came up with but it may not be the best fix. Feel free to fix it in different way.