Change preference type from float to decimal
Change preference type from float to decimal

Fixes #57

AgilTec/spree_store_credits,spree-contrib/spree_store_credits,AgilTec/spree_store_credits,developingchris/spree_store_credits,developingchris/spree_store_credits,spree-contrib/spree_store_credits,developingchris/spree_store_credits,gogo52cn/spree_store_credits,AgilTec/spree_store_credits,gogo52cn/spree_store_credits,gogo52cn/spree_store_credits