Ensure railtie initializer runs before app initializers

In some situations, this was running after the application initializers,
which meant that config for custom fields was not taking effect.

This load-order behaviour seems to depend on what other Engines are
installed, and the order they appear in the Gemfile.
Merge pull request #17 from alphagov/fix_load_order_issue

Ensure railtie initializer runs before app initializers