Pin rails-backbone to ~> 0.9.10 (fixes #7954)

Because of a change in our deployment processes introduced by dpla-attic/automation#95,
rails-backbone was being updated automatically. I'm not sure if it's an issue with
this gem itself or one if its dependencies, but pinning it has the side effect of
also pinning coffee-script, ejs, and jquery-rails.

Gemfile

@@ -9,7 +9,7 @@ group :assets do
   gem 'uglifier', '>= 1.0.3'
   gem 'therubyracer'
   gem 'jquery-rails'
-  gem 'rails-backbone'
+  gem 'rails-backbone', '~> 0.9.10'
 end
 
 gem 'haml'