Fix Migration to work under PostgreSQL

From J. Zellman

Postgres text type does not support an upper limit.

db/migrate/20091021024908_modify_text_types.rb

@@ -1,6 +1,8 @@
 class ModifyTextTypes < ActiveRecord::Migration
   def self.up
-    change_column :revisions, :content, :text, :limit => 16777215
+    unless adapter_name.eql?('PostgreSQL')
+      change_column :revisions, :content, :text, :limit => 16777215
+    end
     change_column :pages, :name, :string, :limit => 255
     change_column :webs, :additional_style, :text
   end