serve previous ibex version on instrument summary page

As a developer I would like the [instrument summary page](http://beamlog.nd.rl.ac.uk/inst_summary.xml) to show the previous version of IBEX which was deployed on a given instrument.

### Acceptance Criteria
_What is the acceptance criteria?_
- [ ] A method of storing the previous version, which gets updated when we deploy, is created
- [ ] this is used to serve the previous version on the summary page

### Extra Information
We used to store this on a wiki page, but would like to automate it as it's one less thing to forget to update.

## How to Test
_verbose instructions for reviewer to test changes
(**Add before making a PR**)_