Allow the app to be run on heroku

Had a request from a frontender to make this easy to test design changes.
They asked for this, and are aware of hoped-for changes in terms of
publishing tools etc.
Meh.

```
2014-07-25T15:44:38.624925+00:00 heroku[api]: Deploy fa0c397 by jabley@example.com
2014-07-25T15:44:38.625032+00:00 heroku[api]: Release v11 created by jabley@example.com
2014-07-25T15:44:43.877305+00:00 app[web.1]: Configuration file: /app/_config.yml
2014-07-25T15:44:44.161677+00:00 app[web.1]:             Source: ./
2014-07-25T15:44:44.387341+00:00 app[web.1]:                     ------------------------------------
2014-07-25T15:44:44.387313+00:00 app[web.1]:              ERROR: YOUR SITE COULD NOT BE BUILT:
2014-07-25T15:44:44.161689+00:00 app[web.1]:        Destination: ./_site
2014-07-25T15:44:44.387385+00:00 app[web.1]:                     Post 0000-00-00-welcome-to-jekyll.markdown.erb does not have a valid date.
2014-07-25T15:44:44.387226+00:00 app[web.1]:       Generating... 
2014-07-25T15:44:45.618067+00:00 heroku[web.1]: State changed from starting to crashed
2014-07-25T15:44:45.619480+00:00 heroku[web.1]: State changed from crashed to starting
2014-07-25T15:44:41.559116+00:00 heroku[web.1]: Stopping all processes with SIGTERM
2014-07-25T15:44:43.543112+00:00 heroku[web.1]: Process exited with status 143
2014-07-25T15:44:42.390501+00:00 heroku[web.1]: Starting process with command `bundle exec jekyll serve --port 48561`
2014-07-25T15:44:45.608073+00:00 heroku[web.1]: Process exited with status 1
```

Will have a look