Fix the migration class name

Due to Active Support auto loading feature, the migration class
shouldn't be name-spaced under the `ActiveStorage` constant, otherwise,
running the migrations would throw an error.

lib/active_storage/migration.rb

@@ -1,4 +1,4 @@
-class ActiveStorage::CreateTables < ActiveRecord::Migration[5.1]
+class ActiveStorageCreateTables < ActiveRecord::Migration[5.1]
   def change
     create_table :active_storage_blobs do |t|
       t.string  :key

test/database/create_users_migration.rb

@@ -1,4 +1,4 @@
-class ActiveStorage::CreateUsers < ActiveRecord::Migration[5.1]
+class ActiveStorageCreateUsers < ActiveRecord::Migration[5.1]
   def change
     create_table :users do |t|
       t.string :name

test/database/setup.rb

@@ -2,5 +2,5 @@
 require_relative "create_users_migration"
 
 ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
-ActiveStorage::CreateTables.migrate(:up)
-ActiveStorage::CreateUsers.migrate(:up)
+ActiveStorageCreateTables.migrate(:up)
+ActiveStorageCreateUsers.migrate(:up)