[Ruby] Exclude test codes from code generation

[ackintosh]

Related issue on kubernetes-client/ruby : kubernetes-client/ruby#29

This is a step for switching to OpenAPI Generator. (#103)

This PR solves the two issues:

• [1] There are not only auto-generated codes but also "hand-written" test codes under spec folder.
  • e.g. spec/config/incluster_config_spec.rb
  • Unintentionally, the hand-written codes are deleted on re-generation
  • To avoid that, we need to exclude the spec folder from cleanup ( 9f57a86 )
• [2] The auto-generated test codes are just skelton
  • so it is unnecessary to generate. ( 8fae10d )

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After this PR has been merged, I would file a PR to kubernetes-client/ruby which cleanup(removes) the auto-generated test codes under spec folder.

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[ackintosh]

I've signed the CLA.

[brendandburns]

/lgtm
/approve

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