export GEM_HOME and GEM_PATH instead of using RVM

https://gist.github.com/thinkerbot/668037 explains this, as follows.

Two ENV variables control the 'gem' command:

    GEM_HOME: the single path to a gem dir where gems are installed
    GEM_PATH: a standard PATH to gem dirs where gems are found

A gem directory is a directory that holds gems.  The 'gem'
command will lay out and utilize the following structure:

    bin               # installed bin scripts
    cache             # .gem files  ex: cache/gem_name.gem
    doc               # rdoc/ri     ex: doc/gem_name/rdoc
    gems              # gem file    ex: gems/gem_name/lib/gem_name.rb
    specifications    # gemspecs    ex: specifications/gem_name.gemspec

As an example of usage:

    export GEM_HOME=a
    export GEM_PATH=a
    gem install rack
    gem list                    # shows rack

    export GEM_HOME=b
    export GEM_PATH=b
    gem install rake
    gem list                    # shows rake (not rack)

    export GEM_PATH=a:b
    gem list                    # shows rake and rack

And if you set GEM_HOME=a:b, you will install into the 'a:b' directory :)