Pipelines and associations APIs are deprecated

[fonji]

Hubspot source

Pipelines and associations APIs currently used by this gem are deprecated.
Sunset is early 2020.

I'm currently working on #120 to update it to the changes made in this gem, so it's the right time to use the new endpoint.
But I don't think I should limit to Deals, and we should implement something to access the associations API.

I guess we need a new Association class, but the relations with other models might be tricky.
If you have an opinion about how to work this out, please let me know.
I'll try to make a PR in the meantime, but I'll limit it to associations.

As I don't use pipelines, can someone else look into it?

[fonji]

#210 takes care of associations.
I don't use the deal pipelines API, so I guess it would be better if someone else manages the upgrade.

[sallesma]

Hi @adimichele

There are several contributions waiting for review. This one seems to have an impact quite soon on projects using the gem.

Is there a chance for it to be reviewed in the upcoming days or weeks ?