Small correction to contributing_to_rails in the section for testing ActiveRecord

Initially the guide suggests to install gems via "bundle install --without db" 
But tests for ActiveRecord need to install the gems in db group also, like mysql.
The patch clears the bundler options in ".bundle/config" and installs all the gems in Gemfile

guides/source/contributing_to_ruby_on_rails.textile

@@ -128,6 +128,12 @@ The test suite of Active Record attempts to run four times: once for SQLite3, on
 
 WARNING: If you're working with Active Record code, you _must_ ensure that the tests pass for at least MySQL, PostgreSQL, and SQLite3. Subtle differences between the various adapters have been behind the rejection of many patches that looked OK when tested only against MySQL.
 
+NOTE: need to install gems in the db group:
+
+<shell>
+$ bundle install --without ""
+</shell>
+
 h5. Database Configuration
 
 The Active Record test suite requires a custom config file: +activerecord/test/config.yml+. An example is provided in +activerecord/test/config.example.yml+ which can be copied and used as needed for your environment.