Fix Page Preview for Comfy 1.8 and Rails 3.2.14

Uses @glebm's fix for the preview button: https://github.com/comfy/comfortable-mexican-sofa/pull/322

Although this was fixed on Comfy 1.9 and Rails 4.0, applications still using Comfy 1.8 and the latest version of Rails 3.2 (3.2.14) have the preview button bug. 

A gem version bump to 1.8.5 for the 1.8 branch would be great.