Combination with SimpleForm's form builder is possible now.
Use "require 'simple_form'" only if ENV['RANSACK_FORM_BUILDER'] is the
SimpleForm builder.
Cleaned-up based on jonatack's suggestions in
https://github.com/activerecord-hackery/ransack/pull/341#issuecomment-36364346
Merge pull request #341 from nagyt234/with_simple_form

Combination with SimpleForm's form builder is possible now.