[Brightbox] Require fog/json in compute

Since `fog-json` was extracted from `fog` it means if you rely on:

    require "fog/brightbox/compute"

`Fog::JSON` was not defined although it was supposed to be a supported
use case. This adds the missing require.

lib/fog/brightbox/core.rb

@@ -1,4 +1,5 @@
 require 'fog/core'
+require 'fog/json'
 
 module Fog
   module Brightbox