How to fix `Gem::ConfigFile::RbConfig (NameError)` in Ruby 1.9.1

I recently had to use a very very old Ruby version for a legacy project. I installed version 1.9.1 via RVM on my Ubuntu 13.04 machine and everything seemed to work great. Until I tried to install the Rails 2.0.2 gem, which produced the following error:

/home/me/.rvm/rubies/ruby-1.9.1-p431/lib/ruby/site_ruby/1.9.1/rubygems/config_file.rb:82:in `rescue in rescue in <class:ConfigFile>':
  uninitialized constant Gem::ConfigFile::RbConfig (NameError)
  from /home/me/.rvm/rubies/ruby-1.9.1-p431/lib/ruby/site_ruby/1.9.1/rubygems/config_file.rb:64:in `rescue in <class:ConfigFile>'
  from /home/me/.rvm/rubies/ruby-1.9.1-p431/lib/ruby/site_ruby/1.9.1/rubygems/config_file.rb:60:in `<class:ConfigFile>'
  from /home/me/.rvm/rubies/ruby-1.9.1-p431/lib/ruby/site_ruby/1.9.1/rubygems/config_file.rb:36:in `<top (required)>'
  from /home/me/.rvm/rubies/ruby-1.9.1-p431/lib/ruby/site_ruby/1.9.1/rubygems/gem_runner.rb:9:in `require'
  from /home/me/.rvm/rubies/ruby-1.9.1-p431/lib/ruby/site_ruby/1.9.1/rubygems/gem_runner.rb:9:in `<top (required)>'
  from /home/me/.rvm/rubies/ruby-1.9.1-p431/bin/gem:13:in `require'
  from /home/me/.rvm/rubies/ruby-1.9.1-p431/bin/gem:13:in `<main>'

I couldn’t really get any info about this issue on the internet, but I discovered that the gem-command can be fixed by adding the line

require 'rbconfig'

to the file on the last line of the stack trace. Just add it right after requiring rubygems and everything should work perfectly.