Add rails 5.0.0 alpha support

[FlowerWrong]

Nowadays, I am using with rails 5 alpha. And I like acts-as-taggable-on. After use it, I found it was not suport rails 5, So I add the suport for it.
The main problem are count and quote_value methods of activerecord.
See https://github.com/rails/rails/blob/master/activerecord/lib/active_record/sanitization.rb#L10 and https://github.com/rails/rails/blob/master/activerecord/lib/active_record/relation/calculations.rb#L38 .
I just add support for them, like this

def count(column_name = :all, options = {})
  ActsAsTaggableOn::Utils.active_record5? ? super(column_name) : super(column_name, options)
end

🎱

[seuros]

We should drop support for rails 3.2 then, that will clean up the codebase a little bit.