Ruby (Rack) application could not be started

[SilverNightFall]

I am getting this error on my website

A source file that the application requires, is missing.

It is possible that you didn't upload your application files correctly. Please check whether all your application files are uploaded.

A required library may not installed. Please install all libraries that this application requires.

Error message:
no such file to load -- bundler/setup
Exception class:
LoadError
Application root:
/var/www/sample_app/current
Backtrace:
# File Line Location
0 /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb 36 in gem_original_require' 1 /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb 36 inrequire'
2 /var/www/sample_app/releases/20120521154812/config/boot.rb 6
3 /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb 36 in gem_original_require' 4 /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb 36 inrequire'
5 /var/www/sample_app/releases/20120521154812/config/application.rb 1
6 /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb 36 in gem_original_require' 7 /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb 36 inrequire'
8 /var/www/sample_app/releases/20120521154812/config/environment.rb 2
9 /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb 36 in gem_original_require' 10 /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb 36 inrequire'
11 config.ru 3
12 /usr/lib/ruby/1.8/rack/builder.rb 46 in instance_eval' 13 /usr/lib/ruby/1.8/rack/builder.rb 46 ininitialize'
14 config.ru 1 in `new'
15 config.ru 1

[indirect]

You don't have Bundler installed. You need to install it.