Add RuboCop to Gemfile

[ackintosh]

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 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.

Description of the PR

closes #2444

[spacether]

@ackintosh @wing328 it looks like Shippable is having problems installing some packages and the right version of Elixir. Per your log:

E: Failed to fetch http://packages.erlang-solutions.com/debian/pool/elixir_1.8.1-2~ubuntu~xenial_amd64.deb  Unable to connect to packages.erlang-solutions.com:http:

Per mine from: https://app.shippable.com/github/OpenAPITools/openapi-generator/runs/6788/1/console
It installed Elixir 1.1.0-dev when we had previously been installing Elixir 1.8.1 per the last successful shippable build here: https://app.shippable.com/github/OpenAPITools/openapi-generator/runs/6786/1/console

[wing328]

The CI failures are not related to this PR.