Only pin minimum versions of our dependencies

django-filer released 0.9.6 a couple days ago and any fresh installation of widgy should use it, also, django_polymorphic 0.5.5 has already been released, so I don't think we need to use the edge version just to install 0.5.1.

BTW, I found a cool pip trick to get a list of all old dependencies:

```sh
$ pip list -o
```

It will show you all of the out-of-date dependencies of your projects.