Upgrade faraday and oauh2 to v2

[marcrohloff]

• Use Faraday 2.0 gem
• Replace custom Faraday GZip implementation with faraday-gzip gem
• Relax gemspec to allow OAuth2 v2.x gem
• No longer support Ruby 2.5
• Remove extraneous pp in specs

Credit goes to @Zajn Original PR https://github.com/ruckus/quickbooks-ruby/pull/58

[ruckus]

Thanks everyone.

So ... this is now the start of a 1+ and 2+ version era. I merged this PR into the new 2-stable branch and released version 2.0 to RubyGems.

Users on Ruby 2.6+ should use 2+ of the gem.

any 1.x versions are legacy and only people still on Ruby 2.5.8 (like me) can only use <= 1.x