Use the django_polymorphic from PyPI.

The 0.5.1 version has been released for a while now, there is no reason
to use the development version anymore.
Specify minimum versions of dependencies.

Instead of pinning exact versions.  Several of our dependencies have
already had several updates and we shouldn't force our users to use old
versions of those packages.
Don't depend on polymorphic any more

Only django-filer needs it, and it already has it as a dependency
Merge pull request #226 from fusionbox/update_filer_dep

Only pin minimum versions of our dependencies