Trigger request via API

@rkh this is what the service class expects as a payload: https://github.com/travis-ci/travis-core/blob/sf-trigger-request/spec/support/payloads.rb#L3-L13

I think you'd then run the service with: 

```ruby
service(:receive_request, event_type: 'push', payload: payload).run
```

@BanzaiMan I like to have it there so I can comment it in when I need it without having to look it up :)