require correct file in migration generator

While executing `rails g flip:install` : 

```
lib/ruby/gems/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require': cannot load such file -- rails/generators/active_record/migration (LoadError)`
```

This PR fixes this by requiring correct file in migration generator. I tested it on rails 4.0.0 and 3.2.15