Use backport of ordereddict for Python 2.6 or earlier.

The 'ImportError' occurs if using Python 2.6 or earlier.

```
.tox/py26/lib/python2.6/site-packages/moto/dynamodb/models.py:1: in <module>
>   from collections import defaultdict, OrderedDict
E   ImportError: cannot import name OrderedDict
```

This patch needs the following backport package for the old version of python.
```
$ sudo pip install ordereddict
```