Make work on centos by installing rvm gem into chef's embedded ruby.

Confirmed this made Centos 6.2 work and confirmed it doesn't break Ubuntu 11.10

Without this fix you'll see this on Centos 6.2:

[2012-09-20T17:06:23-04:00] INFO: *** Chef 10.14.2 ***
[2012-09-20T17:06:24-04:00] INFO: Setting the run_list to ["role[jenkins_slave]"] from JSON
[2012-09-20T17:06:24-04:00] INFO: Run List is [role[jenkins_slave]]
[2012-09-20T17:06:24-04:00] INFO: Run List expands to [jenkins_slave]
[2012-09-20T17:06:24-04:00] INFO: Starting Chef Run for ip-10-252-90-98.us-west-2.compute.internal
[2012-09-20T17:06:24-04:00] INFO: Running start handlers
[2012-09-20T17:06:24-04:00] INFO: Start handlers complete.
[2012-09-20T17:06:24-04:00] INFO: Processing gem_package[rvm] action install (rvm::default line 21)

================================================================================
Recipe Compile Error in /tmp/exec/cookbooks/jenkins_slave/recipes/default.rb
================================================================================

LoadError
---------
cannot load such file -- rvm

Cookbook Trace:
---------------
  /tmp/exec/cookbooks/rvm/recipes/default.rb:27:in `from_file'
  /tmp/exec/cookbooks/rvm/recipes/system_install.rb:20:in `from_file'
  /tmp/exec/cookbooks/rvm/recipes/system.rb:20:in `from_file'
  /tmp/exec/cookbooks/jenkins_slave/recipes/default.rb:4:in `from_file'

Relevant File Content:
----------------------
/tmp/exec/cookbooks/rvm/recipes/default.rb:

  1:  #
  2:  # Cookbook Name:: rvm
  3:  # Recipe:: default
  4:  #
  5:  # Copyright 2010, 2011, Fletcher Nichol
  6:  #
  7:  # Licensed under the Apache License, Version 2.0 (the "License");
  8:  # you may not use this file except in compliance with the License.
  9:  # You may obtain a copy of the License at

[2012-09-20T17:06:25-04:00] ERROR: Running exception handlers
[2012-09-20T17:06:25-04:00] ERROR: Exception handlers complete
[2012-09-20T17:06:25-04:00] FATAL: Stacktrace dumped to /var/chef/cache/chef-stacktrace.out
[2012-09-20T17:06:25-04:00] FATAL: LoadError: cannot load such file -- rvm