sinatraアプリを起動すると、次のエラーメッセージが表示されます。
/home/matt/.rvm/gems/ruby-1.9.3-p125/gems/backports-2.5.1/lib/backports/tools.rb:310:in `require': cannot load such file -- mongomapper (LoadError)
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/backports-2.5.1/lib/backports/tools.rb:310:in `require_with_backports'
from /home/matt/handle-phone-call/app.rb:10:in `<top (required)>'
from /home/matt/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from /home/matt/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from /home/matt/handle-phone-call/config.ru:2:in `block in <main>'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/builder.rb:51:in `instance_eval'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/builder.rb:51:in `initialize'
from /home/matt/handle-phone-call/config.ru:1:in `new'
from /home/matt/handle-phone-call/config.ru:1:in `<main>'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/builder.rb:40:in `eval'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/builder.rb:40:in `parse_file'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/server.rb:200:in `app'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/server.rb:301:in `wrapped_app'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/server.rb:252:in `start'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/server.rb:137:in `start'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/bin/rackup:4:in `<top (required)>'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/bin/rackup:19:in `load'
from /home/matt/.rvm/gems/ruby-1.9.3-p125/bin/rackup:19:in `<main>'
何か案は？どこから始めればいいのかわからない。