Fix Github Actions CI: add rexml gem for ruby 3 & move pry gems into :test group

[mrlevitas]

Issue Type: Bug

Fixes / Relates To: fixes GH Actions CI gem loading bugs on bundle exec rake step:

$ bundle exec rake
WARNING:  No private key present, creating unsigned gem.
rake aborted!
LoadError: cannot load such file -- rexml/parsers/streamparser

as well as:

$ bundle exec rake
WARNING:  No private key present, creating unsigned gem.
rake aborted!
LoadError: cannot load such file -- pry-nav

and

$ bundle exec rake
WARNING:  No private key present, creating unsigned gem.
rake aborted!
LoadError: cannot load such file -- pry-rescue
twitter-ruby-ads-sdk/Rakefile:34:in `<top (required)>'

Changes Included: rexml gem was taken out of ruby in 2.8.0 (https://github.com/googleapis/google-api-ruby-client/issues/871) and ruby:head is newly released 3.0.0, which is missing that gem. Previous versions will already have gem loaded.

for the pry- gems: in Rakefile for CI, bundler doesn't require :development group, so I moved the 2 pry gems into :test group.

if ENV['CI']
  task default: ['spec'] # 'features'
  Bundler.require(:default, :test)

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