Fix Django1.7 warning.

/usr/lib/python2.7/dist-packages/django/forms/utils.py:44:
RemovedInDjango18Warning: In Django 1.8, widget attribute readonly=True
will be rendered as 'readonly'. To preserve current behavior, use the
string 'True' instead of the boolean value.
  RemovedInDjango18Warning

Change-Id: Ia757c43e48ee45e99540c23a3fe78a776fe39139