Upgrade slack-notifier to 1.0.0; use webhook URLs instead of tokens.

Slack's service integration instructions now only show Webhook URL, so
this should be reasonable for new users.

For compatibility, allow an existing pair of `team_name` and
`auth_token` to still work.
Merge pull request #617 from knu/upgrade_slack-notifier

Upgrade slack-notifier to 1.0.0; use webhook URLs instead of tokens.