title: Django 1.7.1 Makemigrations fails when using lambda as default for attribute
body: <p>I'm using Django 1.7.1. My model looks like this:</p>
  
  <pre><code>from datetime import datetime
  from django.db import models
  
  class myModel(models.Model):
      x = models.CharField(max_length=254,null=True, blank=True,)
  </code></pre>
  
  <p>Everything works perfectly fine.</p>
  
  <p>However, when I add the following attribute to myModel, it breaks:</p>
  
  <pre><code>    y = models.DateTimeField(default=lambda: datetime.utcnow() + timedelta(days=1), editable=False)
  </code></pre>
  
  <p><code>manage.py makemigrations</code> gives me the following error: </p>
  
  <pre><code>ValueError: Cannot serialize function: lambda
  </code></pre>
  
  <p>This seems like a known bug: <a href="http://comments.gmane.org/gmane.comp.python.django.scm/125724" rel="noreferrer">http://comments.gmane.org/gmane.comp.python.django.scm/125724</a></p>
  
  <p>So how can I work around it? I need the value of <code>y</code> to be automatically set by default to 24 hours from the moment the model was created.</p>