Improved RubyGems installation.

Enable users to configure the version of RubyGems that will be installed
and also if the system and existing gems have to be upgraded after the
installation.

Also modifies the InstallChefGems statement to allow users to specify
the desired version of the Chef gem to install.
Merge pull request #1386 from nacx/flexible-chef-gems

Flexible Rubygems and Chef installation