using completed_at rather than created_at for more accurate sales total report

Fixes #4205
[backend] Fix fields used in reports_spec test
Bump rabl to 0.9.3

Conflicts:
	api/spree_api.gemspec
[api] Do not run milliseconds precision on MySQL because MySQL does not do milliseconds
[api] Specify stock_items key in variants/index so key is 'stock_items' rather than 'spree_stock_items'
make sure the options are actually created.

Fixes #4271
Fixes #4273

Conflicts:
	backend/app/assets/javascripts/spree/backend/stock_transfer.js.coffee

Conflicts:
	backend/app/assets/javascripts/admin/stock_transfer.js.coffee
[api] Specify products key in products/index so that key is 'products' and not 'spree_products'