Rails 5 - Update subscription.rb to throw abort on callback chain when false is returned

[yuenmichelle1]

Rails 5 deprecated behavior. Update subscription.rb to throw abort on callback chain when false is returned.

See: https://www.bigbinary.com/blog/rails-5-does-not-halt-callback-chain-when-false-is-returned and following deprecation warning on rails 5

DEPRECATION WARNING: Returning `false` in Active Record and Active Model callbacks will not implicitly halt a callback chain in Rails 5.1. To explicitly halt the callback chain, please use `throw :abort` instead. (called from block (4 levels) in <top (required)> at /home/runner/work/talk-api/talk-api/spec/models/subscription_spec.rb:70)

Also adding version constraints on rspec-rails. rspec-rails 5.0 not compatible with rails <= 5.1 but will be compatible with rails 5.2, see https://github.com/rspec/rspec-rails/blob/v5.0.0/Changelog.md#500--2021-03-09