Update rails to 6.0

This updates rails test to the current version. Rails now requires
ActiveRecord classes to have a connection when they are created, so we
can use include ActiveModel::Model to test the same thing.  This also
allows the tests to be run on ruby 2.7 without warnings. The previous
rails version was using a bigdecimal API that doesn't exist anymore.

Gemfile

@@ -3,12 +3,12 @@ source "http://rubygems.org"
 gemspec
 
 group :development, :test do
-  gem 'rake'
   gem 'guard-rspec'
   gem 'simplecov', require: false, platforms: [:mri]
   gem 'coveralls', require: false, platforms: [:mri]
 
-  gem 'activerecord', '~> 4.1'
+  gem 'rake'
+  gem 'activerecord', "~> 6.0"
   gem 'nokogiri', '~> 1.6'
   gem 'ox', '>= 2.1.2', platforms: [:mri, :rbx]
   gem 'oga', '>= 0.3.4'

spec/sax-machine/sax_activerecord_spec.rb

@@ -3,7 +3,9 @@
 
 describe "SAXMachine ActiveRecord integration" do
   before do
-    class MySaxModel < ActiveRecord::Base
+    class MySaxModel
+      include ActiveModel::Model
+
       SAXMachine.configure(MySaxModel) do |c|
         c.element :title
       end