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

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
```