update slack-notifier for new webhook_url format

Created by: stevenosloan

Just got a report about this for the gem and knew y'all were using it so took a crack at getting things updated. Took me a minute to get things running to work on this, so please let me know if I missed anything.

I saw y'all have been running into issues with the way parsing out the token & teams have been working (#gitlab-701), as well as needing to make changes as slack does (https://github.com/gitlabhq/gitlabhq/commit/e5998adc26f66b2b84cbcc28e98f3fc8f89b4696). Talking to their integrations team the preferrence is for integrations to use the raw webhook_url. This gives them more flexibility in the future. (one issue this addressed for them was team names could change w/o breaking webhooks). And should be less headache for integrations if/when they change things.