add rails version dependency in gemspec
add rails version dependency in gemspec

vinsol/fullcalendar-rails-engine,rikkipitt/fullcalendar-rails-engine,vinsol/fullcalendar-rails-engine,ehannes/fullcalendar-rails-engine,rikkipitt/fullcalendar-rails-engine,ehannes/fullcalendar-rails-engine,ehannes/fullcalendar-rails-engine