Bump shoulda-matchers to 4.0.1

This removes the warning when using Ruby 2.6:

BigDecimal.new is deprecated; use BigDecimal() method instead.

This also adds Rails 5 support, which eliminates the need for the monkey
patch to handle
https://github.com/thoughtbot/shoulda-matchers/issues/913.

Gemfile

@@ -379,7 +379,7 @@ group :development, :test do
 end
 
 group :test do
-  gem 'shoulda-matchers', '~> 3.1.2', require: false
+  gem 'shoulda-matchers', '~> 4.0.1', require: false
   gem 'email_spec', '~> 2.2.0'
   gem 'json-schema', '~> 2.8.0'
   gem 'webmock', '~> 3.5.1'

Gemfile.lock

@@ -857,8 +857,8 @@ GEM
     sexp_processor (4.12.0)
     sham_rack (1.3.6)
       rack
-    shoulda-matchers (3.1.2)
-      activesupport (>= 4.0.0)
+    shoulda-matchers (4.0.1)
+      activesupport (>= 4.2.0)
     sidekiq (5.2.7)
       connection_pool (~> 2.2, >= 2.2.2)
       rack (>= 1.5.0)
@@ -1199,7 +1199,7 @@ DEPENDENCIES
   sentry-raven (~> 2.7)
   settingslogic (~> 2.0.9)
   sham_rack (~> 1.3.6)
-  shoulda-matchers (~> 3.1.2)
+  shoulda-matchers (~> 4.0.1)
   sidekiq (~> 5.2.7)
   sidekiq-cron (~> 1.0)
   simple_po_parser (~> 1.1.2)

spec/support/shoulda/matchers/rails_shim.rb

-# monkey patch which fixes serialization matcher in Rails 5
-# https://github.com/thoughtbot/shoulda-matchers/issues/913
-# This can be removed when a new version of shoulda-matchers
-# is released
-module Shoulda
-  module Matchers
-    class RailsShim
-      def self.serialized_attributes_for(model)
-        if defined?(::ActiveRecord::Type::Serialized)
-          # Rails 5+
-          serialized_columns = model.columns.select do |column|
-            model.type_for_attribute(column.name).is_a?(
-              ::ActiveRecord::Type::Serialized
-            )
-          end
-
-          serialized_columns.inject({}) do |hash, column| # rubocop:disable Style/EachWithObject
-            hash[column.name.to_s] = model.type_for_attribute(column.name).coder
-            hash
-          end
-        else
-          model.serialized_attributes
-        end
-      end
-    end
-  end
-end