Fix issue with reaping_frequency type.

When using DATABASE_URL to configure ActiveRecord, reaping_frequency does not get converted from a string to a numeric value.

reaping_frequency is eventually passed to 'sleep' and must be numeric to avoid exceptions.

This PR converts reaping_frequency to a float when present, and alters an existing test to use a string frequency value, confirming that this fix works as intended.

The style utilized is of that found in nearby code as well as on the [Contributing to Ruby on Rails](http://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html) guide.