Replace guzzle with HTTPlug

[ollietb]

Like a lot of other people I'm using other libraries that require Guzzle 5 so I haven't been able to upgrade to Guzzle 6. Now the opportunity has come to upgrade all the libraries I use to the latest Guzzle version, but I didn't want to break backwards compatibility and force people to upgrade if they can't. The best way to go about this is to use an abstraction library, like HTTPlug which will allow users to choose their own version of Guzzle.

I've tried to keep upgrading as simple as possible. For most users it will work out of the box but some changes are unavoidable. They are:

1. The config options for the curl request have been removed and delegated to the Guzzle adapter
2. The event handlers for event.before, event.after and event.error now take different parameters rather than Guzzle Event objects.

@sesser perhaps you want to create a new major version for this as a result of the non-BC breaks above?

[sesser]

I realize this has been sitting here waiting for me to do something and I apologize for ignoring it. Unfortunately, it looks like Facebook is deprecating the old Instagram API in July 2018 and replacing it with a Graph API. I appreciate all the work you have done on this project and if you'd like, I would be more than happy to turn over the project to you along with the domain (it expires tomorrow) if you like.