Revert "Bump aws-sdk to 1.20.0"

This reverts commit c0471ef33faeaa46e1fec943e4cf3526437ee16d.

This version unfairly locks nokogiri to 1.6.0, which may cause issues with Deface.
Ensure that users cannot set price on line items through the API
Fix format of price input in products/new.html.erb
[frontend] Stop people encountering uninitialized Taxon errors when they override ProductsController#show
Remove regression tests for #3404

These were proving extremely difficult to test, and so I have removed them. This issue should no longer come up, anyway.
[api] Remove 'add line items' test from orders controller

if people want to add line items, they should do it through the line items endpoint
Non-GET requests that would usually non-ssl-redirect will now respond with HTTP status 426