Update shipping rate cost column to default to 0.

Fixes #2931
Remove src ||= source from Adjustment#update

This caused the following build failure on spree_auth_devise:
https://gist.github.com/radar/5449670

After removing it, I ran the tests and they all pass:
http://ci.spreecommerce.com/viewLog.html?buildId=6245&buildTypeId=bt5&tab=buildLog#_focus=140

I don't know why this line was ever necessary in the first place. It was added
in a8898232f40116cd72065d265ad997157e278579.
Rely on spree_user mappings from Devise, not user

For reasons, see commit 4a27c35fd29cf4fcafc1cf6b2401d937361610b9 in spree_auth_devise