This PR removes the configuration for Dokku and configures for Heroku deployment, which is the recommended route for deploying a Hubot. It follows Hubot's documentation for Heroku deployment.

This configures plannerbot for Heroku by:

• Removing package-lock.json since having both yarn.lock and package-lock.json is messy (and blocks Heroku from deploying).
• Removing circleci continuous deployment configuration for Dokku.
• Updating documentation to reflect Heroku deploy.

Not reflected in the diff of this PR is the Heroku setup itself, which was done manually:

• Created heroku app labs-plannerbot
• Configured with environment variables
• Installed Redis Cloud add-on (free for 30mb/month)
• Connected to autodeploy from master:
  .

Closes #6 and #8 .