Update respond.js to ver. 1.4.6

- updated respond.js files from original repo to ver. 1.4.6

- updated gem version to match respond.js version

- updated readme, section "CDN/X-Domain Setup"

modified:   README.md
modified:   lib/app/helpers/respond_helper.rb
modified:   lib/respond/rails/version.rb
modified:   respond-rails.gemspec
modified:   vendor/assets/images/respond.proxy.gif
modified:   vendor/assets/javascripts/respond-proxy.html
modified:   vendor/assets/javascripts/respond.js
modified:   vendor/assets/javascripts/respond.proxy.js