Reload shipment after removing variant item via api

Fix broken spec on api build. The item was being removed properly but
the controller still returned the old obj instead of the new one with
the item removed
Force select2-rails to 3.2.1

select2-rails 3.2.2 breaks all capybara helpers in Spree that depends on
the label tag for attribute. e.g.

    <%= label_tag :add_variant_id, Spree.t(:name_or_sku) %>

    Instead of:

    <label for="s2id_add_variant_id">some text</label>

    will generate:

    <label for="s2id_autogen1">some text</label>

see https://github.com/ivaynberg/select2/commit/519070b11da9347cf1542d26fc59a83b0c3fd934

wtf select2 ..

Most of backend request specs were failing due to this
Only reload shipment if it's persisted [Fixes #3037]

Fix remaining backend failing specs
Add index on spree_variants.sku [Fixes #3038]
display stock_transfer number on show page
missing translation for stock transfer show