Housekeeping: gemspec description, push request device dupe, dead test option

[hownowstephen]

Summary

• Updated gemspec description/summary to mention both Track and Transactional APIs (was just "event API")
• Removed device from SendPushRequest::OPTIONAL_FIELDS — it was already mapped to custom_device manually, causing both keys to appear in the message
• Removed no-op json: true option from 5 test Client.new calls (JSON is always the encoding)

Test plan

• All 130 examples pass
• RuboCop clean
• .gitignore already had pkg entry — no change needed

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Note

Low Risk
Low risk housekeeping changes: updates gem metadata text, removes a duplicated push payload field, and simplifies specs by dropping an unused json: true option without altering core request behavior.

Overview
Updates the gemspec description/summary to describe support for the Journeys Track and Transactional APIs (instead of only the event API).

Fixes SendPushRequest payload construction by removing device from OPTIONAL_FIELDS and only mapping opts[:device] into custom_device, avoiding both keys being sent.

Cleans up specs by removing the no-op json: true option from several Customerio::Client.new calls now that JSON encoding is the default.

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