add preferences for gift card expiration

Change-Id: I06bfced8b0ce5340746fe6cbc72e6980aca9d834
use spree preference for expiration date

Change-Id: Ic3a5fbc0dd6281bd0ad6828652936c2af1f7d23a
Merge remote-tracking branch 'upstream/2-1-lav' into 2-1-lav
update default expiration preferences

- update name
- use class method in GiftCard to determine default
- update GiftCard to set expirate_date if it's blank instead of
doing so in controller

Change-Id: I47deb4002660f2da89eec99901bf7b351609fda0
Merge pull request #2 from seantaylor/2-1-lav

Spree Preference for expiration date