Fix #14

I don't know why, but it just works.

I've found that if I move [this line](https://github.com/randym/activeadmin-axlsx/blob/master/lib/active_admin/axlsx.rb#L21) to a separate initializer in my rails app, it will work. So I could just assume that if this line is executed after the initializing of ActiveAdmin, it will probably work. And it did.