Update lib/geocoder/models/mongoid.rb

Doesnt crash if ::Mongoid::VERSION isn't defined.
It does fall back to Mongoid version < 3

lib/geocoder/models/mongoid.rb

@@ -18,7 +18,7 @@ module Geocoder
         super(options)
         if options[:skip_index] == false
           # create 2d index
-          if (::Mongoid::VERSION >= "3")
+          if (defined? ::Mongoid::VERSION && ::Mongoid::VERSION >= "3")
             index({ geocoder_options[:coordinates].to_sym => '2d' }, 
                   {:min => -180, :max => 180})
           else