Lock Rubocop to 0.50

Ville Lautanalarequested to merge
github/fork/lautis/rubocop-fix into master

Rubocop version updates introduce new rules that can break the build. It's not good to fail a build with unrelated changes because a new version of Rubocop was released. As a solution, lock Rubocop to the latest minor version and fix errors.

This caused build failure in https://github.com/intridea/oauth2/pull/318

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