leftover JSON references

[al-bglhk]

I was trying a ruby example upon setting up transactional email when an error came up

[redacted]2.7.6/lib/ruby/gems/2.7.0/gems/customerio-5.1.0/lib/customerio/api.rb:20:in `send_email': uninitialized constant Customerio::APIClient::JSON (NameError)

I dug around and found it could be some leftover that hasn't migrated to multi_json yet. I noticed it's already a dependency of customerio gem.

I changed it on my local and works fine after that.