Fixed so that the migrations work with custom user models

I had the same problem as #205 but updating to the latest version of allauth did not solve it so I implemented the fix suggested in the issue and tested it and it worked.

Under MySQL it worked without any other changes but under PostgreSQL I also had to add the following to the initial migration for the app that contains my custom user model:
```
needed_by = (
    ('socialaccount', '0001_initial'),
)
```
I suppose it might be possible to detect the app that contains the (custom) user model and have a ```depends_on``` in the socialaccount initial migration so that people don't have to make any changes to their migrations, but that seems error-prone so probably not worth doing it.