Broken link to slack channel in readme

[pauloliver]

Clicking the link to the Chart.js slack channel just goes to a page with

Heroku | No such app

[derekperkins]

@tannerlinsley setup the Heroku app originally

[etimberg]

Fixed by @tannerlinsley today

[jacktuck]

Broken again

[simonbrunel]

@tannerlinsley do you still have access to the Heroku app ?

[tannerlinsley]

Unfortunately, I do not.

…

On Wed, Jan 31, 2018 at 10:19 AM Simon Brunel ***@***.***> wrote: @tannerlinsley <https://github.com/tannerlinsley> do you still have access to the Heroku app ? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#3139 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AFUmCSQ8TKYNDMLpE0rdbzrxnKrMy9SHks5tQKCqgaJpZM4Jiiyr> .

[carlstoker]

The link is broken again.

[simonbrunel]

That's because we consumed the allowed quotas for this month, will be automatically fixed in 2 days.