Lock down RuboCop version

Peter Bolingrequested to merge
github/fork/bquorning/lock-rubocop into master

Created by: bquorning

Locks down RuboCop version to ~> 0.52.1 instead of >= 0.37. This should ensure that pull requests don’t run red on Travis whenever RuboCop releases an upgrade.

This PR also trims the .rubocop.yml file a bit. There is no need to specify configuration for a disabled cop, or to specify the same configuration as RuboCop provides by default.

Finally, the -D command line flag has been moved into the configuration file as well, as AllCops[DisplayCopNames] = true.

cc @pboling

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