Account for SocketErrors thrown by Spree::Dash::Jirafe.register

Fixes #3090
Fix documentation regarding in_taxon method

Fixes #3154
add `touch: true` option to Spree::Asset#viewable association

Fixes #3190
Fixing checkout unable to find CouponApplicator class when using threadsafe mode

NameError: uninitialized constant Spree::Core::CouponApplicator was thrown on checkout. Noticed coupon applicator was not being loaded in library.

Fixes #3177
Remove master branch warning from README
Remove 'Overview' link check from backend now that dash has been removed