Review moodlehq/behat-extension dependencies

Seems like some of the requires in the moodlehq/behat-extension composer.json are unnecessary:

• "symfony/symfony" is an odd one, that requires the entire Symfony project. Think we can depend on what Behat requires.
• "guzzlehttp/guzzle" looks unnecessary and isn't directly used in the project. I cannot actually find any of the other projects listing it as a direct dependency.
• "behat/mink", "behat/mink-extension", "behat/mink-goutte-driver", "behat/mink-selenium2-driver" - just wondering if these should be here or in Moodle's composer.json. See behat/symfony2-extension as an example of another Behat extension.

Interestingly, this PR looks like it was going to or is based on a version that addressed some of these problems.