如何解决`require'：无法在heroku上加载这样的文件 - bundler（LoadError）

Question

当我在heroku上运行迁移时，它会给我一个错误

heroku run rake db:migrate
Running rake db:migrate on testing-app... up, run.9199
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- bundler (LoadError)
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from /app/bin/spring:8:in `<top (required)>'
from /app/bin/rake:3:in `load'
from /app/bin/rake:3:in `<main>'