Dont mess unrelated records on CreateItemAdjustments#destroy

Stores could have their whole adjustments history nullified or destroyed
for incompleted orders.

Huge thanks to @athal7 for reporting the issue
Always pass currency to Spree::Money for multi-currency.
Need to only try specifying currency in case order doesnt exist yet.
Update Rails Version in README.md

I just demo-ed Spree the other day for a talk and had to change my rails version from 4.0.4 to 4.0.5 to get everything to work (obviously incredibly minor but I think worth fixing.)
make sure we only build the year when it's there. Validtion error otherwise.

Fixes #4725
Reorder short circuit evaluation of Estimator#shipping_methods

Change-Id: Ifdd04aa7920e5c1033d8c75a526805202be892bc