Revert "Add missing translation for "purchase" [Fixes #32]"

This reverts commit ce3169aae3566db33b29084c5ec8257bf6619e07.

As pointed out by @futhr in
https://github.com/spree/spree_wishlist/commit/ce3169aae3566db33b29084c5ec8257bf6619e07#commitcomment-3344337,
this translation key is not necessary.
Add Spree::Variant#in_any_stock? method

Since stock items and stock locations were added, Spree::Variant's in_stock?
method has been removed.
Maybe such method should be added to spree directly?
Merge remote-tracking branch 'upstream/master'