[Ruby] Fix Ruby client to prevent Rubocop's rule violations (#2100)

Created by: autopp

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @cliffano (2017/07) @zlx (2017/09)

Description of the PR

Fix #2100 (closed). This PR fixes templates of Ruby client to prevent Rubocop's rule violations. (Violations ware mainly Layout/EmptyLinesAroundClassBody and Layout/EmptyLinesAroundModuleBody)

In addition, I also regenerate petstore samples: samples/client/petstore/ruby, samples/openapi3/client/petstore/ruby and samples/client/petstore-security-test/ruby.

$ (cd samples/client/petstore/ruby; rubocop -f o)
 97/97 files |===================== 100 ======================>| Time: 00:00:00 

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0  Total

$ (cd samples/openapi3/client/petstore/ruby; rubocop -f o)
 98/98 files |===================== 100 ======================>| Time: 00:00:00 

--
0  Total

$ (cd samples/client/petstore-security-test/ruby; rubocop -f o)
 15/15 files |===================== 100 ======================>| Time: 00:00:00 

7  Lint/Syntax
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7  Total

Violations remaining in samples/client/petstore-security-test/ruby looks like another issue. I would like to investigate at a later date.