Improved nested update test in update().

Closes #2194.
Improve nested update and create testing.
Renamed validated_attrs to validated_data to be more in line with other similar code.
Merge pull request #2196 from tomchristie/tomchristie-improve-update-nested-validation

Improve checks for nested creates and updates.
Merge pull request #2197 from mtschammer/mtschammer-validated_attrs-rename

Renamed validated_attrs to validated_data to be more in line with other code
Updated Django versions to match requirements
Removed custom python2_unicode_compatible. Closes #2138 (thanks @sh4r3m4n)
Merge branch 'master' of github.com:tomchristie/django-rest-framework
Removed custom StringIO, force_text, smart_text compat