Force Sidekiq to encode all jobs as ASCII-only.

This gets around the problem of making sure the app and the worker are
both using the same (or at least compatible) encoding settings, by
forcing all the JSON to be ASCII-compatible.
Stop YAJL from overriding the `JSON` constant.

YAJL doesn't support the `ascii_only` option on `generate`, which we're
using when patching Sidekiq.
Merge pull request #139 from alphagov/fix-json-job-encoding-2-fix-harder

Fix JSON job encoding… again.