Add configurable json_adapter

In some cases the JSON adapter MultiJson chooses is inappropriate.

This commit adds a configuration paramter :json_adapter.

Setting the json_adapter configuration option to one of the adapters
MultiJson knows about, e.g. :json_gem, will allow raven to use that
JSON adapter.

Change-Id: I8306f45707c7016ac5f7feb1147587d557dee8fa
Reviewed-on: https://gerrit.causes.com/23397
Reviewed-by: Lann Martin <lann@causes.com>
Reviewed-by: John Skopis <john.skopis@causes.com>
Tested-by: John Skopis <john.skopis@causes.com>