This affects the cache key version (updated to 7) and introduces
a dependency on `ujson` which is BSD licenced as of the version
locked in `requirements.txt`

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

Update cache keys to use JSON

See merge request !1151

Rather than constantly using regular expressions and retrieval from YAML
nodes, pre-parse expansion strings into a list representation cached
for reuse, and then expand them as simple string concatenation.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>