Add support for max-duration capability option for saucelabs.

The selenium2 config section allowed the definition of max-duration, but not saucelabs.  It's unclear why
max-duration was rewritten to max_duration before the behat.yml config was validated, but this change
works if you use max-duration in your behat.yml config, e.g:

    default:
      extensions:
        SilverStripe\BehatExtension\MinkExtension:
          saucelabs:
            capabilities:
              max-duration: 900