rvm_gem options don't work, should allow a string argument like gem_package

I wanted to install a gem without dependencies like so:
```ruby
rvm_gem "foo" do
  version "0.7.0"
  options "--ignore-dependencies"
end
```
but it would not work, I tried using a hash like `options(:ignore_dependencies=>true)` but this just got mangled into `ignoredependenciestrue` in the generated command.

Chef's own `gem_package` task supports either a String or a Hash, so I changed the allowed types for `rvm_gem`'s `options` attribute.  Now they can be either a String or a Hash, and testing with a string (as above) it is now working for me.