Add Spree 2.1 functionality

**As a** developer 
**In order to** add promotions by taxon with Spree  2.1 
**I want** _Spree Taxon Promo_ to be compatible with Spree 2.1 

**In order to** be able to work big amount of taxons 
**I want** _Spree Taxon Promo_ to have autocomplete fields for taxons in promotion rule block and adjustment. (Admin section)