Updated Silex support + added Pagerfanta Twitter Bootstrap 3 View support

Hi,

This is a small update to your wonderful silex-pagerfanta-provider. Mainly consists of two components:

1. With the latest `Silex 1.1+`, your current `composer.json` that requires `symfony/proper-access: 2.2.*@dev` will  remove the native `symfony/proper-access: 2.4.1` version, causing some issues. I have updated it so that it now allows higher installed version.

2. `Pagerfanta` added `TwitterBootstrap3View` sometime ago. I added a few lines in the `PagerfantaServiceProvider.php` to enable the support for this new view. 

Cheers. :)