Bug 1277593 - Make FlexibleForeignKey compatible with Django 1.9
Bug 1277593 - Make FlexibleForeignKey compatible with Django 1.9

Since `related_field` was renamed to `target_field` in:
https://github.com/django/django/pull/4241

Fixes:
AttributeError: 'FlexibleForeignKey' object has no attribute 'related_field'

akhileshpillai/treeherder,akhileshpillai/treeherder,glenn124f/treeherder,kapy2010/treeherder,tojonmz/treeherder,tojon/treeherder,akhileshpillai/treeherder,tojonmz/treeherder,kapy2010/treeherder,glenn124f/treeherder,tojonmz/treeherder,KWierso/treeherder,glenn124f/treeherder,edmorley/treeherder,kapy2010/treeherder,tojon/treeherder,KWierso/treeherder,kapy2010/treeherder,tojonmz/treeherder,tojonmz/treeherder,akhileshpillai/treeherder,edmorley/treeherder,glenn124f/treeherder,KWierso/treeherder,KWierso/treeherder,edmorley/treeherder,tojon/treeherder,akhileshpillai/treeherder,tojon/treeherder,kapy2010/treeherder,glenn124f/treeherder,akhileshpillai/treeherder,glenn124f/treeherder,tojonmz/treeherder,edmorley/treeherder