Quick Bundler
Quick Bundler searches for the nearest Gemfile and loads the gems specified inside it. It's nothing more than a wrapper to Bundler to locate a Gemfile, load it, then require all gems in the Gemfile.
Installing
gem install quick-bundler
Usage
Stick this at the top of you're app ...
require "rubygems"
require "quick-bundler"
... and you're done!