Calum/deletes old docs take 2

@carloslima so I found the source of the bug. Even though my Mock model has the _timestamp field defined in its _mapping, I didn't actually apply this mapping to the ES index in the spec. When ES indexed the docs, it didn't know to assign a timestamp to them, hence the failure. ES couldn't delete any docs based on the value of their timestamp field, because none of the docs *had* a timestamp field! :D

I have removed the sleep from MockData#import, as it's not necessary.

You can now go ahead and review/merge this :-)