Version substitution 724

Fixes #724

I have tested this by setting `version = '0.1.2-99-ge08b10f'` in `conf.py` and seeing an error:

``gettingstarted/tutorial/vagrant-setup.rst:202:ge:vagrant-setup``

without this fix, and no error with this fix.
This class is unnecessary. You could just do in `conf.py`:

```
spelling_filters = [lambda tokenizer: IgnoreWordsFilter(tokenizer, [version])]
```