Serialize enum values correctly for migrations

As reported by @bh
> @hannseman 
> ```python
class Gender(enum.Enum):
    MALE = 1
    FEMALE = 2
    UNKNOWN = 3
class Foo(models.Model):
    gender  = enum.EnumField(Gender)
```
I get a new issue when running:
$ python manage.py makemigrations
```
  File "/home/bhe/.virtualenvs/mx-py34/lib/python3.4/site-packages/django/db/migrations/writer.py", line 303, in serialize
    raise ValueError("Cannot serialize: %r\nThere are some values Django cannot serialize into migration files.\nFor more, see https://docs.djangoproject.com/en/dev/topics/migrations/#migration-serializing" % value)
ValueError: Cannot serialize: FEMALE
There are some values Django cannot serialize into migration files.
```