Add :send_core_emails config prefernce

Allow developers to use standard rails mail configuration (in config
files) by setting :override_actionmailer_config to false without sending
spree core emails (e.g. order confirmation). This is useful e.g. in the
case where devs have opted to use an external mail API such as Mandrill
for store-related emails but still want to use ActionMailer in other
parts of their app.

Fixes #3812
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.