Bump our Backbone dependency to 1.6.
Review Request #13573 — Created Feb. 26, 2024 and submitted — Latest diff uploaded
Backbone 1.6 came out a few weeks ago. It contains some minor bug fixes
that don't really impact Spina. We, however, had also missed the 1.5
release, which also didn't impact us but was a bigger release.This change catches us up to the latest 1.6.
Unit tests pass.