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.
Merge pull request #44 from stof/allow_raw_goutte

Changed the typehint to allow using a normal Goutte client