Not working without Mongoid

[Ilya Lavrov]

Current version generate exception when app did not require 'Mongoid' gem.

NameError (uninitialized constant Rails3JQueryAutocomplete::Autocomplete::ClassMethods::Mongoid):
/Users/ilavrov/.bundler/ruby/2.0.0/rails3-jquery-autocomplete-554dbb2e8b19/lib/rails3-jquery-autocomplete/autocomplete.rb:44

[Joiey Seeley]

Fixed by #281

Thanks for the pull request.