Changed the typehint to allow using a normal Goutte client

The extended client shipped with the driver was implemented to detect the content-type based on the HTML meta tag when the header is not set. As the feature has been merged in DomCrawler in Symfony 2.2.7, it makes sense to allow passing a normal Goutte instance now.

When moving to Mink 2.0, the extended client class will be removed, as BrowserKitDriver 2.0 will require BrowserKit 2.3+, so we can require DomCrawler 2.3+ too.
In the 1.x branch, I'm keeping the extended client for the default instantiation to be sure to preserve BC for people who don't build the client explicitly.