undefined method `constantize' for Resque:Module Did you mean? constants

I am getting undefined method constantize' for Resque:Module Did you mean? constantserror while clicking onSchedulefromresquepage i.e/resque/schedule` I started getting this error few days back. I haven't changed anything in the code.

Below is the gem code https://github.com/resque/resque-scheduler/blob/v2.0.0.g/lib/resque_scheduler/server.rb#L17

I am using below gems for resque:

gem 'resque', '~> 1.25.0', :require => 'resque/server'
gem 'resque-fairly', '~> 1.1.0'
gem 'resque-scheduler', '2.0.0.g'
gem 'rufus-scheduler', '2.0.24'
gem 'resque-retry', '0.2.2'
gem 'resque-lock', '1.0.0'

I tried to update the resque-scheduler version >= 2.0.1 , then the Schedule button disappears.

I am using ruby 2.3.0 and rails 3

Below is the error image