[DRE-maint] Bug#918613: ruby-rack-contrib FTBFS with ruby-rack 2.0.6-2

[Adrian Bunk]

Source: ruby-rack-contrib
Version: 1.3.0-3
Severity: serious
Tags: ftbfs buster sid

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/ruby-rack-contrib.html

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usr/lib/ruby/2.5.0/rubygems/dependency.rb:312:in `to_specs': Could not find 'rack' (~> 1.4) - did find: [rack-2.0.6] (Gem::MissingSpecVersionError)
...