turn off Rubocop::Cop::Style::WhileUntilModifier
turn off Rubocop::Cop::Style::WhileUntilModifier

The modifier while/until construction should not be preferred.

The style guide has been updated to reflect this:
  https://gollum.instructure.com/Ruby-style-guidelines#While/Until

Change-Id: Ib1b8b3cf29d52729ba9d3f9066554fc421d47111
Reviewed-on: https://gerrit.instructure.com/52997
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins
Product-Review: Strand McCutchen <3926e7f2370f1e7814576250688185c598b36b91@instructure.com>
QA-Review: Strand McCutchen <3926e7f2370f1e7814576250688185c598b36b91@instructure.com>

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