remove dependency on oauth2 1.4 gem version which is EOL

[bretweinraub]

Installing oauth2 gem 1.4.x now throws the following:

You have installed oauth2 version 1.4.10, which is EOL.
No further support is anticipated for the 1.4.x series.

OAuth2 version 2 is released.
There are BREAKING changes, but most will not encounter them, and upgrading should be easy!

Please see:
• https://github.com/oauth-xx/oauth2#what-is-new-for-v20
• https://github.com/oauth-xx/oauth2/blob/master/CHANGELOG.md

I have upgrade to 2.0 and via rspec, no new tests fail.

I still get this failure, but that already exists in ruckus/quickbooks-ruby master branch:

1) Quickbooks VERSION 
   Failure/Error: subject { Quickbooks::VERSION }
 
   NameError:
     uninitialized constant Quickbooks::VERSION
   # ./spec/lib/quickbooks_spec.rb:4:in `block (3 levels) in <top (required)>'
   # ./spec/lib/quickbooks_spec.rb:5:in `block (3 levels) in <top (required)>'
   # /Users/bweinraub/.rvm/gems/ruby-2.7.2@quickbooks-ruby-ruckus/gems/webmock-3.14.0/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'

[ashkulz]

@bretweinraub can you retarget the PR to the 2-stable branch, as this change makes sense to happen there?