Use correct migration class in acceptance tests

In Rails 5.0, migration classes changed so that they were versioned:
instead of inheriting from `ActiveRecord::Migration`, you inherited from
`ActiveRecord::Migration[5.0]`. The old way wasn't removed, however --
that is, until Rails 5.1. Hence, our acceptance tests that use the old
style no longer work under the 5.1 Appraisal.

spec/acceptance/multiple_libraries_integration_spec.rb

@@ -5,7 +5,7 @@ describe 'shoulda-matchers integrates with multiple libraries' do
     create_rails_application
 
     write_file 'db/migrate/1_create_users.rb', <<-FILE
-      class CreateUsers < ActiveRecord::Migration
+      class CreateUsers < #{migration_class_name}
         def self.up
           create_table :users do |t|
             t.string :name

spec/acceptance/rails_integration_spec.rb

@@ -5,7 +5,7 @@ describe 'shoulda-matchers integrates with Rails' do
     create_rails_application
 
     write_file 'db/migrate/1_create_users.rb', <<-FILE
-      class CreateUsers < ActiveRecord::Migration
+      class CreateUsers < #{migration_class_name}
         def self.up
           create_table :users do |t|
             t.string :name

spec/support/acceptance/helpers.rb

@@ -3,6 +3,7 @@ require_relative 'helpers/base_helpers'
 require_relative 'helpers/command_helpers'
 require_relative 'helpers/gem_helpers'
 require_relative 'helpers/n_unit_helpers'
+require_relative 'helpers/rails_migration_helpers'
 require_relative 'helpers/rails_version_helpers'
 require_relative 'helpers/rspec_helpers'
 require_relative 'helpers/ruby_version_helpers'
@@ -23,6 +24,7 @@ module AcceptanceTests
     include CommandHelpers
     include GemHelpers
     include NUnitHelpers
+    include RailsMigrationHelpers
     include RailsVersionHelpers
     include RspecHelpers
     include RubyVersionHelpers

spec/support/acceptance/helpers/rails_migration_helpers.rb

@@ -0,0 +1,21 @@
+require_relative 'gem_helpers'
+
+module AcceptanceTests
+  module RailsMigrationHelpers
+    include RailsVersionHelpers
+
+    def migration_class_name
+      if rails_version >= 5
+        "ActiveRecord::Migration[#{rails_version_for_migration}]"
+      else
+        'ActiveRecord::Migration'
+      end
+    end
+
+    private
+
+    def rails_version_for_migration
+      rails_version.to_s.split('.')[0..1].join('.')
+    end
+  end
+end