Use standard way of generating migrations in engines

Keep tenant column:

Until we merge the feature that always turns on tenancy
(#79), we must keep
the dummy Rails app's migrations disjoint from what
`rake plutus:migrations:install` actually copies over

README.markdown

@@ -16,9 +16,7 @@ Installation
 ============
 
 - Add the gem to your Gemfile `gem "plutus"`
-
-- generate migration files `rails g plutus`
-
+- generate migration files `rake plutus:install:migrations`
 - run migrations `rake db:migrate`
 
 Overview

lib/generators/plutus/templates/migration.rb → db/migrate/20160422010135_create_plutus_tables.rb

@@ -1,5 +1,5 @@
 class CreatePlutusTables < ActiveRecord::Migration[4.2]
-  def self.up
+  def change
     create_table :plutus_accounts do |t|
       t.string :name
       t.string :type
@@ -30,10 +30,4 @@ def self.up
     add_index :plutus_amounts, [:account_id, :entry_id]
     add_index :plutus_amounts, [:entry_id, :account_id]
   end
-
-  def self.down
-    drop_table :plutus_accounts
-    drop_table :plutus_entries
-    drop_table :plutus_amounts
-  end
 end

fixture_rails_root/db/migrate/20170710173916_create_plutus_tables.rb

@@ -1,5 +1,5 @@
 class CreatePlutusTables < ActiveRecord::Migration[4.2]
-  def self.up
+  def change
     create_table :plutus_accounts do |t|
       t.string :name
       t.string :type
@@ -30,10 +30,4 @@ def self.up
     add_index :plutus_amounts, [:account_id, :entry_id]
     add_index :plutus_amounts, [:entry_id, :account_id]
   end
-
-  def self.down
-    drop_table :plutus_accounts
-    drop_table :plutus_entries
-    drop_table :plutus_amounts
-  end
 end

fixture_rails_root/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170710174915) do
+ActiveRecord::Schema.define(version: 20180709082453) do
 
   create_table "plutus_accounts", force: :cascade do |t|
     t.string "name"