Require arel master in the benchmark template

Pointing at Rails master generally requires pointing at Arel master

guides/bug_report_templates/benchmark.rb

@@ -8,6 +8,7 @@ end
 gemfile(true) do
   source "https://rubygems.org"
   gem "rails", github: "rails/rails"
+  gem "arel", github: "rails/arel"
   gem "benchmark-ips"
 end