Remove the json dependency

[jfeltesse-mdsol]

JSON has been included in ruby since 1.9:

$ irb -f
irb(main):001:0> RUBY_VERSION
# => "1.9.3"
irb(main):002:0> require "json"
# => true
irb(main):003:0> JSON
# => JSON

Forcing everyone to use a recent version like 2.1+ is a good thing but sometimes it may be detrimental: some older apps may still be stuck on 1.x for some reason and currently they wouldn't be able to use generated clients because of the 2.1+ requirement.
Unless there's a specific reason to use 2.1+ in the clients, listing (or not) json as a dependency should be up to the user of the generated clients.

For example, rails dropped the explicit dependency a long time ago (PR).

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

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