Explicitly Require JSON Library

[lessthanjacob]

Checklist:

• I have updated the necessary documentation
• I have signed off all my commits as required by DCO
• My build is green

Background

Fixes #443

There's been a longstanding implicit dependency on the standard json library, as Blueprinter::Configuration requires it to be available in the case where a separate JSON library isn't specified on configuration. This hasn't caused many issues in the past, as most use cases for Blueprinter are in environments where json is loaded (e.g. a Rails application).

However, this is a brittle dependency, and if we're explicitly defining a default, we should make sure it's available in the confines of the class that needs it.

Changelog

• Explicitly require 'json' within Blueprinter::Configuration.
• Remove require 'json' from specs.

[jhollinger]

👍