Spec boilerplate

@angusbayley In response to your email

If you run `bundle exec rspec spec` in the project root, you should be able to get up and running quite quickly.

I've set up a framework of sorts which you can work with, some methods and specs are yet to be written - to be solved by the reader!

Essentially, the sinatra `app.rb` file now just calls a few well defined services (see [here](https://github.com/angusbayley/slack-trustpilot-integration/blob/spec-boilerplate/app.rb)). The app itself isn't tested (this can be done later) but the services each have simple spec files, for example see [service](https://github.com/angusbayley/slack-trustpilot-integration/blob/spec-boilerplate/lib/slack_notifier.rb) and [service spec](https://github.com/angusbayley/slack-trustpilot-integration/blob/spec-boilerplate/spec/lib/slack_notifier_spec.rb).