Merge branch 'fix-services-doc-generation' into 'master'

Fix `rake services:doc` generation

From: https://github.com/gitlabhq/gitlabhq/pull/9604#issuecomment-137539926

See merge request !1243

doc/development/rake_tasks.md

@@ -27,3 +27,9 @@ You can find results under the `doc/code` directory.
 ```
 bundle exec rake gitlab:generate_docs
 ```
+
+## Generate API documentation for project services (e.g. Slack)
+
+```
+bundle exec rake services:doc
+```

lib/tasks/services.rake

@@ -44,7 +44,7 @@ DELETE /projects/:id/services/<%= service[:dashed_name] %>
 ERB
 
 namespace :services do
-  task :doc do
+  task doc: :environment do
     services = Service.available_services_names.map do |s|
       service_start = Time.now
       klass = "#{s}_service".classify.constantize