Revert "Merge pull request #113 from xuanxu/remove_relation"

This reverts commit 9978fc40a8, reversing
changes made to b454667574.

lib/arel.rb

@@ -12,6 +12,8 @@ require 'arel/attributes'
 require 'arel/compatibility/wheres'
 
 #### these are deprecated
+# The Arel::Relation constant is referenced in Rails
+require 'arel/relation'
 require 'arel/expression'
 ####
 

lib/arel/relation.rb

@@ -0,0 +1,6 @@
+module Arel
+  ###
+  # This is deprecated.  Fix rails, then remove this.
+  module Relation
+  end
+end

lib/arel/tree_manager.rb

@@ -1,5 +1,7 @@
 module Arel
   class TreeManager
+    # FIXME: Remove this.
+    include Arel::Relation
     include Arel::FactoryMethods
 
     attr_reader :ast, :engine