[OP-7726] untar ruby version as rbenv user/group

when installing a ruby version from a pre-compiled tarball, untar it as the 
rbenv user. This will allow subsequent gem installs to work properly as the 
rbenv user

fixes this error:

```
    * zendesk_rbenv_ruby[2.1.3] action install[2014-10-21T02:06:23+00:00] INFO: Processing zendesk_rbenv_ruby[2.1.3] action install (/var/chef/cache/cookbooks/zendesk_base/providers/project_scaffold.rb line 60)
 (up to date)
    * rbenv_gem[gem bundler-1.6.1 for 2.1.3] action install[2014-10-21T02:06:26+00:00] INFO: Processing rbenv_gem[gem bundler-1.6.1 for 2.1.3] action install (/var/chef/cache/cookbooks/zendesk_base/providers/project_scaffold.rb line 62)


      ================================================================================
      Error executing action `install` on resource 'rbenv_gem[gem bundler-1.6.1 for 2.1.3]'
      ================================================================================

      Mixlib::ShellOut::ShellCommandFailed
      ------------------------------------
      Expected process to exit with [0], but received '1'
      ---- Begin output of /opt/rbenv/versions/2.1.3/bin/gem install bundler -q --no-rdoc --no-ri  -v "1.6.1"  ----
      STDOUT:
      STDERR: ERROR:  While executing gem ... (Errno::EACCES)
          Permission denied @ rb_sysopen - /root/.gem/ruby/2.1.0/cache/bundler-1.6.1.gem
      ---- End output of /opt/rbenv/versions/2.1.3/bin/gem install bundler -q --no-rdoc --no-ri  -v "1.6.1"  ----
      Ran /opt/rbenv/versions/2.1.3/bin/gem install bundler -q --no-rdoc --no-ri  -v "1.6.1"  returned 1

      Cookbook Trace:
      ---------------
      /var/chef/cache/cookbooks/zendesk_rbenv/libraries/provider_rbenv_rubygems.rb:88:in `install_via_gem_command'
      /var/chef/cache/cookbooks/zendesk_rbenv/libraries/provider_rbenv_rubygems.rb:71:in `install_package'

      Resource Declaration:
      ---------------------
      # In /var/chef/cache/cookbooks/zendesk_base/providers/project_scaffold.rb

       62:         zendesk_rbenv_gem "gem #{name}-#{ver} for #{rubie}" do
       63:           package_name name
       64:           ruby_version rubie
       65:           version ver
       66:         end
       67:       end

      Compiled Resource:
      ------------------
      # Declared in /var/chef/cache/cookbooks/zendesk_base/providers/project_scaffold.rb:62:in `block (3 levels) in class_from_file'

      zendesk_rbenv_gem("gem bundler-1.6.1 for 2.1.3") do
        provider Chef::Provider::Package::RbenvRubygems
        action :install
        retries 0
        retry_delay 2
        guard_interpreter :default
        cookbook_name "zendesk_classic_app"
        ruby_version "2.1.3"
        version "1.6.1"
        package_name "bundler"
        gem_binary "/opt/rbenv/shims/gem"
      end

[2014-10-21T02:06:30+00:00] INFO: Running queued delayed notifications before re-raising exception

    ================================================================================
    Error executing action `create` on resource 'zendesk_base_project_scaffold[zendesk]'
    ================================================================================

    Mixlib::ShellOut::ShellCommandFailed
    ------------------------------------
    rbenv_gem[gem bundler-1.6.1 for 2.1.3] (/var/chef/cache/cookbooks/zendesk_base/providers/project_scaffold.rb line 62) had an error: Mixlib::ShellOut::ShellCommandFailed: Expected process to exit with [0], but received '1'
    ---- Begin output of /opt/rbenv/versions/2.1.3/bin/gem install bundler -q --no-rdoc --no-ri  -v "1.6.1"  ----
    STDOUT:
    STDERR: ERROR:  While executing gem ... (Errno::EACCES)
        Permission denied @ rb_sysopen - /root/.gem/ruby/2.1.0/cache/bundler-1.6.1.gem
    ---- End output of /opt/rbenv/versions/2.1.3/bin/gem install bundler -q --no-rdoc --no-ri  -v "1.6.1"  ----
    Ran /opt/rbenv/versions/2.1.3/bin/gem install bundler -q --no-rdoc --no-ri  -v "1.6.1"  returned 1

    Cookbook Trace:
    ---------------
    /var/chef/cache/cookbooks/zendesk_rbenv/libraries/provider_rbenv_rubygems.rb:88:in `install_via_gem_command'
    /var/chef/cache/cookbooks/zendesk_rbenv/libraries/provider_rbenv_rubygems.rb:71:in `install_package'

    Resource Declaration:
    ---------------------
    # In /var/chef/cache/cookbooks/zendesk_classic_app/recipes/default.rb

     16: zendesk_base_project_scaffold "zendesk" do
     17:   rubies node["zendesk_classic_app"]["rubies"]
     18:   gems node["zendesk_classic_app"]["gems"]
     19: end
     20:

    Compiled Resource:
    ------------------
    # Declared in /var/chef/cache/cookbooks/zendesk_classic_app/recipes/default.rb:16:in `from_file'

    zendesk_base_project_scaffold("zendesk") do
      action :create
      updated true
      updated_by_last_action true
      retries 0
      retry_delay 2
      guard_interpreter :default
      cookbook_name "zendesk_classic_app"
      recipe_name "default"
      rubies ["2.0.0-p353", "2.1.1", "2.1.2", "2.1.3"]
      gems {"bundler"=>"1.6.1"}
    end
```

`gem install` tries to copy the gem into the following two paths as the rbenv user:
```
  - GEM PATHS:
     - /opt/rbenv/versions/2.1.3/lib/ruby/gems/2.1.0
     - /root/.gem/ruby/2.1.0
```

both these directories are owned by `root`, and the gem install errors out after
trying both options as the `rbenv` user

@zendesk/chef