[RFC] Allow Event Dispatcher versions greater than 2.1

Hi,

Thanks very much for the library.

My application runs on Symfony 2.3 LTS and the current composer.json requirement for the event dispatcher is `"symfony/event-dispatcher": "~2.1"`. This is problematic as it only allows versions from the `2.1` branch. Being on Symfony 2.3 I cannot install newer versions of Solarium.

I changed the dependency to allow for Symfony Event Dispatchers *above* 2.1 and it appears working in my application on the `2.3` branch and I just ran your tests using the Event Dispatcher `2.4` branch and the tests passed (results below).

Is it possible to change this dependency? I went through your open issues and I see no mention of why only the `2.1` branch of the Event Dispatcher is allowed.

Thank you very much. 

```
~ composer install --dev
Loading composer repositories with package information
Installing dependencies (including require-dev)
  - Installing symfony/event-dispatcher (v2.4.2)
    Downloading: 100%

  - Installing symfony/yaml (v2.4.2)
    Downloading: 100%

  - Installing phpunit/php-text-template (1.2.0)
    Loading from cache

  - Installing phpunit/phpunit-mock-objects (1.2.3)
    Downloading: 100%

  - Installing phpunit/php-timer (1.0.5)
    Downloading: 100%

  - Installing phpunit/php-token-stream (1.2.1)
    Loading from cache

  - Installing phpunit/php-file-iterator (1.3.4)
    Loading from cache

  - Installing phpunit/php-code-coverage (1.2.15)
    Loading from cache

  - Installing phpunit/phpunit (3.7.31)
    Loading from cache

  - Installing squizlabs/php_codesniffer (1.5.2)
    Downloading: 100%
```

```
PHPUnit 3.7.31 by Sebastian Bergmann.

Configuration read from /var/www/audiobookmart/vendor/solarium/solarium/phpunit.xml.dist

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