Route task to proper queue

In #4033, I introduce a bug when using Celery signatures.

From Celery's docs at http://docs.celeryproject.org/en/latest/reference/celery.html#celery.signature

> the .s() shortcut does not allow you to specify execution options
  but there’s a chaning .set method that returns the signature

So, instead of dealing with multiple `.set()`, I'm just using the
`.signature()` method of the task which is more explicit.

Author: humitos

readthedocs/core/utils/__init__.py

@@ -139,13 +139,12 @@ def prepare_build(
     options['time_limit'] = int(time_limit * 1.2)
 
     update_docs_task = UpdateDocsTask()
-
-    # Py 2.7 doesn't support ``**`` expand syntax twice. We create just one big
-    # kwargs (including the options) for this and expand it just once.
-    # return update_docs_task.si(project.pk, **kwargs, **options)
-    kwargs.update(options)
-
-    return update_docs_task.si(project.pk, **kwargs)
+    return update_docs_task.signature(
+        (project.pk,),
+        immutable=True,
+        kwargs=kwargs,
+        options=options,
+    )
 
 
 def trigger_build(project, version=None, record=True, force=False):