Handle situation when `expect` is executed inside of abstract class.

By mistake I removed my branch for previous PR https://github.com/BossaConsulting/phpspec2-expect/pull/16

Problem occurs for classes that extend abstract class which executes `expect` function. 
In my case it's behat's context file with some default steps.

How to reproduce:

```php
<?php

include 'vendor/autoload.php';

abstract class MyAbstract
{
    public function run()
    {
        expect(true)->toBe(true);
    }
}

class MyClass extends MyAbstract
{
}

$obj = new MyClass();
$obj->run();

// PHP Fatal error:  Cannot instantiate abstract class MyAbstract in /vagrant/vendor/bossa/phpspec2-expect/expect.php on line 76
```
ping @cakper (git blame revealed [you wrote this part](https://github.com/whisller/phpspec2-expect/commit/1acf4e7504278121ab3248affd705ac4c4076567#diff-ffb3b7078c4e1b3bfb4d3ac7461e9dc6R59)).

Why's this not simply `$object = $trace[1]['object']`?