Make AssetUrlProcessor opt-in instead of opt-out

The introduction of the `AssetUrlProcessor` broke folks' assets when
they upgraded sprockets-rails, and it was not released in a new major
version. To remedy this, let's make `AssetUrlProcessor` be opt-in for
now (i.e. in sprockets-rails v3.x) with the plan to change it to opt-out
in the next major version (v4.0).

Author: rmacklin

lib/sprockets/railtie.rb

@@ -103,7 +103,7 @@ def configure(&block)
     config.assets.prefix           = "/assets"
     config.assets.manifest         = nil
     config.assets.quiet            = false
-    config.assets.rewrite_css_urls = true
+    config.assets.rewrite_css_urls = false # This will default to `true` in sprockets-rails v4
 
     initializer :set_default_precompile do |app|
       if using_sprockets4?