Update gem.rb

Added string as an accepted input type for the "options" attribute.

I did this when trying to pass options for installing nokogiri and couldnt understand how to properly follow the installers instructions on error:

use: gem install nokogiri -- --use-system-libraries

With the change in this pull request the following works perfectly:

rbenv_gem "nokogiri" do
  ruby_version "1.9.3-p545"
  options '-- --use-system-libraries'
end