Revert "require the constants we use. ensure that root always returns a Pathname"

This reverts commit d77b576c03.

railties/lib/rails/engine/configuration.rb

@@ -1,5 +1,4 @@
 require 'rails/railtie/configuration'
-require 'pathname'
 
 module Rails
   class Engine
@@ -9,7 +8,7 @@ module Rails
 
       def initialize(root=nil)
         super()
-        @root = root && Pathname.new(root)
+        @root = root
         @generators = app_generators.dup
       end