Calculate previous migration version in specs support

This makes it possible to test migration on the schema this migration
was written for, without a need to specify a previous schema version
manually.

spec/migrations/migrate_build_stage_reference_spec.rb

 require 'spec_helper'
 require Rails.root.join('db', 'post_migrate', '20170526185921_migrate_build_stage_reference.rb')
 
-describe MigrateBuildStageReference, :migration, schema: 20170526185602 do
+describe MigrateBuildStageReference, :migration do
   ##
   # Create test data - pipeline and CI/CD jobs.
   #

spec/migrations/migrate_pipeline_stages_spec.rb

 require 'spec_helper'
 require Rails.root.join('db', 'post_migrate', '20170526185842_migrate_pipeline_stages.rb')
 
-describe MigratePipelineStages, :migration, schema: 20170526185602 do
+describe MigratePipelineStages, :migration do
   ##
   # Create test data - pipeline and CI/CD jobs.
   #

spec/spec_helper.rb

@@ -94,10 +94,10 @@ RSpec.configure do |config|
     Sidekiq.redis(&:flushall)
   end
 
-  config.around(:example, migration: true) do |example|
+  config.around(:example, :migration) do |example|
     begin
-      schema_version = example.metadata.fetch(:schema)
-      ActiveRecord::Migrator.migrate(migrations_paths, schema_version)
+      ActiveRecord::Migrator
+        .migrate(migrations_paths, previous_migration.version)
 
       example.run
     ensure

spec/support/migrations_helpers.rb

@@ -11,6 +11,16 @@ module MigrationsHelpers
     ActiveRecord::Base.connection.table_exists?(name)
   end
 
+  def migrations
+    ActiveRecord::Migrator.migrations(migrations_paths)
+  end
+
+  def previous_migration
+    migrations.each_cons(2) do |previous, migration|
+      break previous if migration.name == described_class.name
+    end
+  end
+
   def migrate!
     ActiveRecord::Migrator.up(migrations_paths) do |migration|
       migration.name == described_class.name