Replace probot-config with context.config

[GeorgianaElena]

The probot-config repository has been archived and implemented in probot itself (ref: probot/probot#975).

This PR:

• removes probot-config and replaces it with context.config
• bumps the probot version to a version that includes probot-config
  • replace deprecated getContent with getContents
  • replace deprecated .event prop with .name
    (ref: v9.0.0 breaking changes)

I think this will also solve #48.

cc @hiimbex

[welcome]

Thanks so much for opening your first PR here!

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would update some of our documentation based on your changes.

[GeorgianaElena]

The tests are passing 🎉, so this should be ready for review.