Integrate with new Slack API endpoint (Breaking Change)

As the Incoming Webhook URL for new Slack integrations has changed to a
different URL.

This is a breaking change - `:slack_subdomain` and `:slack_token` values
are no longer required, instead we must set `:slack_url`, which can be
generated for each integration by following the Incoming Webhook links
from https://api.slack.com

    set :slack_url,
'https://hooks.slack.com/services/your/integration/url'

This closes #11