title: column "date" cannot be cast automatically to type timestamp with time zone django/postgres
body: <p>There were a <code>date</code> charfield , when I saved date as string before. But now I changed the field from charfield to datetimefield and dropped all previous data. </p>
  
  <pre><code>date = models.DateTimeField(default=timezone.now)
  </code></pre>
  
  <p>Localy it works fine, but in production it raises following error when trying to migrate . I dropped the column , the model, commented and uncommented, deleted and created the field again, returned to previous migrations and migrated again, but all that didn't help.</p>
  
  <pre><code>    django.db.utils.ProgrammingError: column "date" cannot be cast automatically to type timestamp with time zone
  HINT:  You might need to specify "USING date::timestamp with time zone".
  </code></pre>
  
  <p>How to solve the problem correctly? Using django 1.8, python 2.7 and postgres</p>