Several stracktraces like this today, so did the needful and changed the 'get' to
a filter and return the first Instance instead.

Traceback (most recent call last):

  File "/home/ubuntu/srv/formhub-ec2/project_env/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 115, in get_response
    response = callback(request, *callback_args, **callback_kwargs)

  File "/home/ubuntu/srv/formhub-ec2/formhub/odk_logger/views.py", line 599, in view_download_submission
    user__username=username, deleted_at=None)

  File "/home/ubuntu/srv/formhub-ec2/project_env/local/lib/python2.7/site-packages/django/shortcuts/__init__.py", line 113, in get_object_or_404
    return queryset.get(*args, **kwargs)

  File "/home/ubuntu/srv/formhub-ec2/project_env/local/lib/python2.7/site-packages/django/db/models/query.py", line 407, in get
    (self.model._meta.object_name, num))

MultipleObjectsReturned: get() returned more than one Instance -- it returned 2!
Changed the response to 404 if the filter slice not found (since that was the original expected behavior)
Not a real commit: the last commit failed b/c travis could not access formhub.org,
which is a problem the way the tests were written.

Anyway, this commit is just a way to try travis again now (7pm EDT) when there is less traffic and all the tests should pass (sad, but there is little alternative).
Merge pull request #1334 from SEL-Columbia/fix_download_submission_properly

Fix download submission properly

this triggers a pull request that failed because it looks for this file from formhub.org. Merge now and raise an issue with test being fragile

ref: test that passed is at 
https://travis-ci.org/SEL-Columbia/formhub/builds/30506674