Revert "Bump Django version to 1.7c3"

This reverts commit e89c8dc238fae2c81c69d3992717ccec160d9a80.
Testing speed up: bulk insert Predictions

3 test cases were inserting several thousand Prediction objects one at a
time. This does bulk insertion. See timing results below:

```
real    4m17.027s
real    0m28.400s
```

Result! [Delivers #76460256]