explicitly load activerecord/activesupport

When used in conjunction with Bundler, ActiveRecord will already be
available in the environment, and joiner can access the ActiveRecord and
ActiveSupport objects.

When used completely outside Bundler, ActiveRecord and ActiveSupport are
not yet loaded, so joiner cannot be used.

This allows the following command to succeed:

  ruby -e "require 'joiner'"
Merge pull request #4 from ktdreyer/activerecord-deps

explicitly load activerecord/activesupport