Update client.py

Hi praekelt!
We have a problem with current language.

```
class SupportForm(BootstrapForm):
    # other fields
    captcha = ReCaptchaField(label=_(u'Are you human?'), attrs={'lang': get_language(), 'theme': 'white'}
    )
```

On production server we have the same message
![recaptcha-error](https://f.cloud.github.com/assets/380957/1333902/b5b2f764-359e-11e3-827f-2803494e1933.jpg)

Example pages:
- [ru page](http://postmanbot.com/ru/support/)
- [en page](http://postmanbot.com/en/support/)

We offer in [string](https://github.com/praekelt/django-recaptcha/blob/master/captcha/client.py#L51-L52)

```
    if not 'lang' in attrs:
        attrs['lang'] = settings.LANGUAGE_CODE[:2]
```
change to
```
    if not 'lang' in attrs:
        attrs['lang'] = get_language()[:2]
```
Switched to json module from standard library in accordance with depreciation of django.utils.simplejson in Django 1.5.
fix django-recaptcha for django 1.6
fix import
fix import