I ran into the same problem. I fixed it in a stretch request: https://github.com/GetJobber/recurring_select/pull/115
Basically, at the moment this stone only supports Rails 3 and Rails 4, I just added a simple condition so that it supports Rails 5 with the same functionality as Rails 4.
You can use the branched branch until it is accepted by adding it to your gemfile:
gem 'recurring_select', git: 'https://github.com/sahild/recurring_select.git', branch: 'master'