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: I2c9865289dbf10a401902014a20ae05516057ee0