Temporarily ignore CVE-2020-8184 in rack while waiting on fix (#4391)

We are blocked from upgrading `rack` because of a dependency conflict
with Sidekiq5 (which is in the long process of being upgraded in #XXXX)

While we get a fix in for this specific issue in rack through a patch,
we want to avoid breaking the build for the next day or so until a fix
is implemented. This PR temporarily ignores the vulnerability check for
this CVE.

rakelib/security.rake

@@ -14,7 +14,7 @@ task security: :environment do
 
   puts 'running bundle-audit to check for insecure dependencies...'
   exit!(1) unless ShellCommand.run('bundle-audit update')
-  audit_result = ShellCommand.run('bundle-audit check --ignore CVE-2020-8161')
+  audit_result = ShellCommand.run('bundle-audit check --ignore CVE-2020-8161 CVE-2020-8184')
 
   puts "\n"
   if brakeman_result && audit_result