Add explicit dependency on Faraday < 2.0

[Zajn]

Faraday made some fairly significant changes in the 2.0
update. quickbooks-ruby currently uses a Faraday middlware, which
was deprecated in the 2.0 update. This adds an explicit
dependency on Faraday < 2.0 rather than using whatever Faraday version
the oauth-2 gem installs.

Fixes #570

[ruckus]

Thanks @Zajn - much appreciated.

[diogogallo]

Hello @Zajn / @ruckus any plans to upgrade to Faraday > 2.0?

[TomA-R]

This would be really good, otherwise we're stuck on Faraday < 2. Is this on the roadmap @ruckus ?

[ruckus]

@TomA-R I think/hope/pray that supporting of Faraday > 2 is as simple as:

• removing the current gzip middleware as a whole. Probably not a big deal at the end of the day. Or, find a suitable replacement. But probably simpler to just yank it.
• switch to the Faraday > 2 model for HTTP Basic auth currently used
  here:

https://github.com/ruckus/quickbooks-ruby/blob/master/lib/quickbooks/service/access_token.rb#L25

in favor of the new helpers discussed here:

https://lostisland.github.io/faraday/middleware/authentication

The reality is that at this juncture I don't have the time to look into this. But if someone were to take a stab at this then I hope it's as simple as just addressing the above 2 items.

[Zajn]

@ruckus @TomA-R I opened #578 to upgrade Faraday.

All tests passed on my branch locally, but I wasn't sure what else needed to be tested, so I didn't add anything. If you have time to give it a look and see anything that you think should be tested, let me know and I can try and add something later this week.