Replace deprecated MultiJson constant with JSON.parse

[maximn]

What

Replace the deprecated MultiJson constant with JSON.parse in the generated client.

Why

The multi_json gem in its 1.21.x line deprecated the CamelCase MultiJson constant in favor of MultiJSON (all-caps), and will remove it entirely in v2.0. As a result, simply loading platform-api now emits a deprecation warning on every Ruby process:

The MultiJson constant is deprecated and will be removed in v2.0. Use MultiJSON instead.

This affects every downstream consumer of platform-api — the warning is emitted whether or not you ever call the Heroku API in your process.

Approach

The generator (heroics) has already fixed this upstream:

• heroics 0.1.3 template emits MultiJson.load(...)
• heroics 0.1.4 template emits JSON.parse(...) and removed its multi_json dependency entirely (CHANGELOG)

The platform-api gemspec already allows heroics ~> 0.1.1, so the long-term fix is just bundle update heroics && rake build. However, that produces a noisy ~1,900-line whitespace-only diff (heroics 0.1.4 also tweaked template whitespace), which is hard to review.

This PR applies the single substantive change — MultiJson.load → JSON.parse — directly so the deprecation warning goes away immediately without the whitespace churn. The next time someone runs rake build against heroics >= 0.1.4, the regenerated file will produce the same line.

Compatibility

• JSON.parse is part of the Ruby standard library — no new dependency.
• Behavior is identical: both calls parse the embedded JSON schema literal at gem load time.
• No API changes for consumers.

[maximn]

Thanks for the review! That block is a verbatim quote of the deprecation warning emitted by multi_json itself — the gem renamed its own constant from MultiJson to MultiJSON (all-caps), and the warning points users at the new name. Quoting it accurately is intentional so the message is searchable.

The PR's fix is different (and stronger) than what the warning suggests: rather than switching to MultiJSON, we drop the multi_json dependency entirely and use stdlib JSON.parse. That's covered in the "Approach" section below. Happy to add a clarifying sentence if it's confusing — let me know.