Allow loading fixture data

This is now working - see my email for details.
Curb requires native extensions which weren't building on the build env. Was simpler to replace with a library that doesn't need compilation (and Excon is a great library regardless). I described the motivation in the commit: https://github.com/ministryofjustice/accelerated_claims/commit/24c532ff82ff2634998da51d247506b7066137fd