Allow AR::Schema's migrations_path to be overwritten by subclasses

Extracts the hard-coded migrations path so it can be customized by subclasses. This is useful when you have a separate schema and migrations using a different connection. Otherwise, the default path of 'db/migrate' causes assume_migrated_up_to make the wrong assumption about prior migrations.

class SignalId::Schema < ActiveRecord::Schema
  def self.migrations_path
    SignalId.root + '/db/migrate'
  end
end

Ideally, we'd have something like a SchemaLoader that took a schema and a migration path. Since Schema is a Migration, Migrator could reasonably assume this responsibility as well. But that would be incompatible with normal usage which is simiply load 'schema.rb'.

Patch applies to master and 2-3-stable.