Added a safeguard for the Goutte version in MinkGoutteDriver

MinkGoutteDriver 1.2 will be compatible with both Goutte 1.x and Goutte
2.x, but MinkExtension 1.3 only supports configuring Goutte 1.x. Upgrading
to Goutte 2.x will be possible only when using MinkExtension 2.0.