Change init order for sprockets-rails 3.0 compat.

It seems that sprockets-rails 3.0.0 does not initialize the `app.assets` object
until it runs its `after_initialize` block. This means that xray-rails must move
its initialization process into `after_initialize` as well.

The exception is the `config.assets.precompile` modifications, which must be
done earlier before that array gets frozen.

These changes fix the "asset pipeline is currently disabled" error when starting
up a Rails app that uses sprockets-rails 3.0.0.

Author: mattbrictson

lib/xray/engine.rb

@@ -7,11 +7,14 @@ module Xray
   # xray.js and the xray bar into the app's response bodies.
   class Engine < ::Rails::Engine
     initializer "xray.initialize" do |app|
-      ensure_asset_pipeline_enabled! app
       app.middleware.use Xray::Middleware
 
       # Required by Rails 4.1
       app.config.assets.precompile += %w(xray.js xray-backbone.js xray.css)
+    end
+
+    config.after_initialize do |app|
+      ensure_asset_pipeline_enabled! app
 
       # Register as a Sprockets processor to augment JS files, including
       # compiled coffeescript, with filepath information. See