@root_install should be @system_install

I believe this fixes a typo in the rvmrc.erb template, judging by the code in `chef_rvm_recipe_helpers.rb`:

```
t = template rvmrc_file do
          source      "rvmrc.erb"
          owner       opts[:user] || "root"
          mode        "0644"
          variables   :system_install   => system_install,
          ...
```