RubyGems 2.0 compat: use #empty? to check for results

The return value of `Gem::DependencyInstaller#find_gems_with_sources` has 
changed in RubyGems 2.0+. Previously an `Array` was returned and now a 
`Gem::AvailableSet` is returned. While the `#any?` method does not exist 
on both of these types the `#empty?` does!


This fixes GH-6.