Provide a way to specify gem processing order

Otherwise, you can have dependency issues, such as using mislav-will_paginate and rails 2.0.2 with rails > 2.0.2 installed.

See this app to reproduce the error:

http://github.com/jtrupiano/gemi...

Solution is probably to preserve order of gems, but this is trickier with multiple config files merged. See config_builder.rb.

Workaround is to explicitly activate proper version of rails before invoking autogem:

require 'rubygems'
gem 'geminstaller', ">= 0.4.5"
require 'geminstaller'
gem 'rails', '= 2.0.2'
GemInstaller.autogem('--exceptions')