Minor changes

Changed the spree_core version to ~>2.0.0 and removed the require lines from the css/js assets.
Hope it's ok.

Might try extending it to add Time selection.
So Admin can specify the slot period (1 period = 30 mins or 1 period = 60mins, etc).
Then the customer can select the time he/she would like the items delivered (available slots could probably be based on 'zones'. This idea still needs some thought, but might be useful for stores like supermarkets that deliver perishable goods.

If you have time to work on a feature like this it would be awesome. Either way, thanks for the extension! Great work! :p