django.utils.simplejson is deprecated in Django 1.5

The [Django 1.5 release notes](https://docs.djangoproject.com/en/dev/releases/1.5/#system-version-of-simplejson-no-longer-used) state that:

> "Django 1.5 deprecates django.utils.simplejson in favor of Python 2.6’s built-in json module."

This causes `django-recaptcha` to raise an exception when run under Django 1.7 (development snapshot from Git):

    ...

    File "/usr/local/lib/python2.7/dist-packages/captcha/fields.py", line 9, in <module>
      from captcha import client

    File "/usr/local/lib/python2.7/dist-packages/captcha/client.py", line 6, in <module>
      from django.utils import simplejson as json

    ImportError: cannot import name simplejson

My pull request simply replaces the import of `django.utils.simple_json` with an import of `json` from the standard library.