Fixes 'newrelic_rpm' gem install in ruby-agent recipe

According to Chef docs, using `gem_package` is the same as using `Chef::Provider::Package::Rubygems`; please see [gem_package resource](http://docs.opscode.com/resource_gem_package.html) for verification.

Using Chef 11.12.2, this recipe raises an error stating that `Chef::Provider::Package::RubyGem` is an undefined constant (**notice RubyGem is singluar!**). Perhaps the provider name recently changed in this version of Chef. Regardless, I believe `gem_package` should be used because it abstracts the provider class name.