Explicitly provide action :install in chef_gem call

Fixes: Error executing action `install` on resource 'chef_gem[rvm]'
>  executing action `install` on resource 'chef_gem[rvm]'
> ** [out :: 10.0.1.2] NoMethodError
> ** [out :: 10.0.1.2] -------------
> ** [out :: 10.0.1.2] undefined method `full_name' for nil:NilClass