JenkinsDeprecatedService#execute delegated to nil service_hook

/cc: @ayufan, @grzesiek

https://sentry.gitlap.com/gitlab/gitlabcom/issues/12/

NoMethodError: undefined method `execute' for nil:NilClass
  app/models/project_services/jenkins_deprecated_service.rb:23:in `execute'
    delegate :execute, to: :service_hook, prefix: nil
  app/workers/project_service_worker.rb:8:in `perform'
    Service.find(hook_id).execute(data)
  lib/gitlab/sidekiq_middleware/memory_killer.rb:17:in `call'
    yield
  lib/gitlab/sidekiq_middleware/arguments_logger.rb:6:in `call'
    yield
  lib/gitlab/metrics/sidekiq_middleware.rb:11:in `block in call'
    trans.run { yield }
...
(25 additional frame(s) were not displayed)

Module::DelegationError: JenkinsDeprecatedService#execute delegated to service_hook.execute, but service_hook is nil: #<JenkinsDeprecatedService id: 1986613, type: "JenkinsDeprecatedService", title: nil, project_id: 348977, created_at: "2015-08-24 10:26:59", updated_at: "2015-08-24 10:30:08", active: true, properties: {"project_url"=>"jenkins.baadgames.com/job/bs-frontend"}, template: false, push_events: true, issues_events: true, merge_requests_events: true, tag_push_events: true, note_events: true, build_events: false, category: "ci", default: false, wiki_page_events: true>
  app/models/project_services/jenkins_deprecated_service.rb:23:in `rescue in execute'
    delegate :execute, to: :service_hook, prefix: nil
  app/models/project_services/jenkins_deprecated_service.rb:23:in `execute'
    delegate :execute, to: :service_hook, prefix: nil
  app/workers/project_service_worker.rb:8:in `perform'
    Service.find(hook_id).execute(data)
  lib/gitlab/sidekiq_middleware/memory_killer.rb:17:in `call'
    yield
  lib/gitlab/sidekiq_middleware/arguments_logger.rb:6:in `call'
    yield
...
(26 additional frame(s) were not displayed)