Corrected module name

Corrected module name, there is no `python-simplejson`:

```
Downloading/unpacking python-simplejson (from -r requirements.txt (line 4))
Could not find any downloads that satisfy the requirement python-simplejson (from -r requirements.txt (line 4))
No distributions at all found for python-simplejson (from -r requirements.txt (line 4))
```

The right name is `simplejson`.