Use webhook url for slack notifier.

Recently slack changed the way incoming webhooks are handled. Instead of taking a team name and token, they now provide a unique (obfuscated) webhook url. And then, [slack-notifier](https://github.com/stevenosloan/slack-notifier), which is dependency package of this, already have changed at https://github.com/stevenosloan/slack-notifier/commit/04006c8d2aeba4bf9086ff2524bae88919a4d424 and released v1.0.0.

I have changed `ExceptionNotifier::SlackNotifier` and it is compatible the way that use webhook url.