Adding symfony 2.0-2.1 requirement in composer.json

Hello,

Here the same pull Request for the 2.0-2.1 symfony requirements.

If someone tries to require this branch with a wrong symfony version, composer will display this message :
<pre>
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for divi/ajax-login-bundle dev-composer-2.0 -> satisfiable by divi/ajax-login-bundle[dev-composer-2.0].
    - divi/ajax-login-bundle dev-composer-2.0 requires symfony/symfony >=2.0,<2.2 -> no matching package found.
</pre>

Regards,
Jérôme