大家好，我在往 heroku 上部署应用的时候，报错了。奋斗了好长时间无果.唉...

Pushing to [email protected]:gentle-water-2643.git
Counting objects: 507, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (449/449), done.
Writing objects: 100% (507/507), 540.76 KiB | 111 KiB/s, done.
Total 507 (delta 122), reused 0 (delta 0)

-----> Heroku receiving push
-----> Ruby/Rails app detected
-----> Installing dependencies using Bundler version 1.1.rc.7
       Running: bundle install --without development:test --path vendor/bundle --binstubs bin/ --deployment
       You are trying to install in deployment mode after changing
       your Gemfile. Run `bundle install` elsewhere and add the
       updated Gemfile.lock to version control.
       You have added to the Gemfile:
       * will_paginate
 !
 !     Failed to install gems via Bundler.
 !
 !     Heroku push rejected, failed to compile Ruby/rails app

To [email protected]:gentle-water-2643.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to '[email protected]:gentle-water-2643.git'

我的 Gemfile 里，我有写：

gem 'will_paginate', :group => [:development, :production]

为什么报错呢。