Remove validation of `attrs` in _get_request_object

Validation is performed downstream, so this call is redundant. In
addition, `attrs` can also now be a list which supports the new DataView
facets format.
Change test DataView json structures to use the new facet format

This suppresses the deprecation warnings emitted by Avocado.
Add support for setting the `context_json` on set instances

If the model has a `context_json` field, this will allow persisting
the original context that was used to create the set. Note, this only
applies to new instances, so the original context is not overwritten.

Fix #159
Fix requirements.txt to use the master branch of Avocado

Update 1.4.x and 1.5.x Django point versions
Merge pull request #160 from cbmi/issue-159

Add support for setting the `context_json` on set instances