We check the database router in order to determine if we want to handle

evolutions for a particular model. The code that does this today checks

for certain attributes on the router and then calls a function. However,

this isn't quite good enough, as the function signature even for

Django's migrations capability has changed twice.
We now have a couple compatibility functions that check whether models

are allowed to migrate (Django >= 1.7) or be synced to the database

(Django <= 1.6). These handle the different function signatures we have

to deal with.
Unit tests are not added in this change, but will be here as part of a

set of signature function test rewrites.

Unit tests pass on supported versions of Django.