Trouble starting rails application

I am very new to ruby on rails and I'm having this problem with creating a new controller for my "plans" resource:

rails generate controller plans
/Users/RentaroSatomi/.rbenv/versions/2.1.5/lib/ruby/gems/2.1.0/gems/bundler-1.8.0/lib/bundler/runtime.rb:34:in 
'block in setup': You have already activated spring 1.3.3, 
but your Gemfile requires spring 1.3.1. 
Prepending 'bundle exec' to your command may solve this. (Gem::LoadError)

The error says something about having a wrong spring version, but I really don't know how to fix this.