Fix: builds for ruby 3+

[ylecuyer]

Current builds for ruby 3+ are failing because of this error:

Failures:

  1) Koala::Facebook::GraphAPIMethods #graph_call the appsecret_proof option is enabled by default if an app secret is present
     Failure/Error: response = api(path, args, verb, options)

       #<Koala::Facebook::API:0x000056012ae8[25](https://github.com/arsduo/koala/runs/6555738033?check_suite_focus=true#step:5:26)c0 @access_token="*", @app_secret="mysecret"> received :api with unexpected arguments
         expected: ("/path", {}, "get", {:appsecret_proof=>true})
              got: ("/path", {}, "get", {:appsecret_proof=>true})
     # ./lib/koala/api.rb:47:in `graph_call'
     # ./spec/cases/graph_api_spec.rb:61:in `block (4 levels) in <top (required)>'

As far as I know this is linked to latest rspec updates and we need rspec/rspec-support#537 to be merged and released before we can use latest versions of rspec

For the moment I'm locking the rspec version to the latest one which was working flawlessly