Fix rubocop violation, lock rack to avoid dependency madness

Signed-off-by: Steven Danna <steve@chef.io>

Gemfile

@@ -34,6 +34,8 @@ group :tools do
   gem 'rb-readline'
   gem 'license_finder'
   gem 'github_changelog_generator', '~> 1'
+  # To avoid ruby version requirements
+  gem "rack", "< 2.0"
 end
 
 # gems for Maintainers.md generation

lib/inspec/dependencies.rb

@@ -157,7 +157,7 @@ module Inspec
     #
     # @param [Object] dependency
     # @return [Boolean] whether this dependency can safely be skipped.
-    def allow_missing?(dependency)
+    def allow_missing?(_dependency)
       # TODO
       false
     end