Use correct index syntax for mongoid 2.x on business support facets

These facet classes were cloned from an application which ran mongoid 3.x.
The index definitions use a different syntax between mongoid 2 and 3 so fix this.

For reference here's the original commit on Imminence where the indexes were updated:
https://github.com/alphagov/imminence/commit/ae9b6ba261ced8eb2671b8df3b0ee93b3122f2ad#diff-c19b3a2bed3e63b1e2b66b5776f9d351L6