Add the option to use a proc to modify the data being sent by a webhook

[TikiTDO]

I have a use case where I would like to post a notification to a webhook, but I want to control the data being posted. This implements a process_data argument on the Webhook handler. This argument can be set to a proc data, which receives all 5 of the notify arguments, and returns the object that will be posted/sent as json.

[TikiTDO]

There appears to be an issue with the version of bundler that travis-ci uses, which I reported here. The actual code should pass all tests when these issues are sorted out.