Fix datafix:install generator in Rails 4

Apparently there was an API change between rails 3 and 4 that obviated the need
to extend ActiveRecord::Generators::Migration onto
Datafix::Generators::InstallGenerator. The install task fails with:

    [WARNING] Could not load generator "generators/datafix/install/install_generator". Error: uninitialized constant ActiveRecord::Generators::Migration.

This correct this issue, with all specs passing.