Use dev key when config.developer_mode = true

[artinboghosian]

There are endpoints on leanplum api that allow setting of devMode: true in the request. The library adds devMode based on config.developer_mode setting. However, when you call track_multi, the production connection is always used. This results in a 403 error from the api: {"success"=>false, "error"=>{"message"=>"Development mode requested but not permitted."}}. On endpoints where devMode can be set the library should use the proper connection based on config.developer_mode setting.