add missing arel dependency to active_record_master bug report script.

[ci skip].

This solves:

```
Could not find gem 'arel (~> 5.0.0) ruby', which is required by gem 'rails (>= 0) ruby', in any of the sources.
```

guides/bug_report_templates/active_record_master.rb

@@ -2,6 +2,7 @@ unless File.exist?('Gemfile')
   File.write('Gemfile', <<-GEMFILE)
     source 'https://rubygems.org'
     gem 'rails', github: 'rails/rails'
+    gem 'arel', github: 'rails/arel'
     gem 'sqlite3'
   GEMFILE