Pin mongoid-locker to 2.0.0

The front-office and back-office apps when the engine has been bumped have also taken on the latest version of mongoid-locker (v2.0.1) because it was not pinned.

Though only a minor change it appears it has a dependency on Mongoid v7 and changes it has made around not using ActiveSupport method `delegate` since version 7.1

We ascertained this from a statement made in the issue [mongoid 7.1.0 issue with arguments](mongoid/mongoid-locker#85 (comment))

> Mongoid gem doesn't use ActiveSupport method delegate since 7.1 version.

The issue was fixed in the mongoid-locker PR [Add forwardable module and upgrade rubocop](mongoid/mongoid-locker#86). And that change seems to be the key difference between version 2.0.0 and 2.0.1.

The error we see in the apps when v2.0.1 is being used is

```
An error occurred while loading ./spec/forms/waste_carriers_engine/base_forms_spec.rb.
Failure/Error: delegate :contact_address, :contact_email, :registered_address, to: :registration
ArgumentError:
  wrong number of arguments (given 4, expected 1)
```

In this example if you go to order_copy_cards_registration.rb:10 you'll find

```ruby
    delegate :contact_address, :contact_email, :registered_address, to: :registration
```

Depending on whether you run rspec or try to hit a page in the app this is the error that gets thrown.

So we are pinning the version of mongoid-locker we use, until we can complete the work to upgrade the version of ruby and rails we are using. We should then be able to use Mongoid v7.1 (or greater) and hopefully the issue goes away!

Gemfile.lock

@@ -12,7 +12,7 @@ PATH
       jbuilder (~> 2.0)
       jquery-rails
       mongoid (~> 5.2.0)
-      mongoid-locker
+      mongoid-locker (= 2.0.0)
       nokogiri
       rails (~> 4.2.11)
       rest-client (~> 2.0)
@@ -165,7 +165,7 @@ GEM
     method_source (1.0.0)
     mime-types (3.3.1)
       mime-types-data (~> 3.2015)
-    mime-types-data (3.2020.0425)
+    mime-types-data (3.2020.0512)
     mini_mime (1.0.2)
     mini_portile2 (2.4.0)
     minitest (5.14.1)
@@ -176,7 +176,7 @@ GEM
       mongo (>= 2.4.1, < 3.0.0)
       origin (~> 2.3)
       tzinfo (>= 0.3.37)
-    mongoid-locker (2.0.1)
+    mongoid-locker (2.0.0)
       mongoid (>= 5.0, < 8)
     multi_json (1.14.1)
     multipart-post (2.1.1)

waste_carriers_engine.gemspec

@@ -22,8 +22,10 @@ Gem::Specification.new do |s|
   s.add_dependency "rails", "~> 4.2.11"
   # Use MongoDB as the database
   s.add_dependency "mongoid", "~> 5.2.0"
-  # Implement document-level locking
-  s.add_dependency "mongoid-locker"
+  # Implement document-level locking. Pinned to 2.0.0 because we believe 2.0.1
+  # has a dependency on using Mongoid version 7 or greater. On v5 we get errors
+  # regards the use of `delegate` when using 2.0.1
+  s.add_dependency "mongoid-locker", "2.0.0"
   # Use jquery as the JavaScript library
   s.add_dependency "jquery-rails"
   # Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks