Use Gem API

With "system" call, install fail when you are not using default gem in path.

For example, installation fail with chef and vagrant:
Vagrant install its own ruby and make it default PATH.
Chef install its own ruby but make it default only with a gem_environment.

with current implementation:

    /opt/chef/embedded/bin/gem install_specific

install in:

    /opt/vagrant_ruby/lib/ruby/gems/1.8/gems/

instead of:

    /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/